Force SMTP server disconnect

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Force SMTP server disconnect

Diego Castillo
Hi all,

I would like to force my Postfix SMTP server to disconnect from time to
time, in order to prevent its clients from reusing the same connection for
too long or for too many transactions.

After reading through the documentation and searching the archives of this
mailing list, I can not find any configuration parameter for doing this. I
have just found this parameter
[http://www.postfix.org/postconf.5.html#smtpd_hard_error_limit] for forcing
server disconnection after some errors. Is it possible to set up similar
limits, but based on time or number of SMTP transactions?

Thank you,


Diego Castillo

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Re: Force SMTP server disconnect

Wietse Venema
Diego Castillo:
> Hi all,
>
> I would like to force my Postfix SMTP server to disconnect from time to
> time, in order to prevent its clients from reusing the same connection for
> too long or for too many transactions.
>
> After reading through the documentation and searching the archives of this
> mailing list, I can not find any configuration parameter for doing this. I

This project takes pride in providing accurate documentation. If
something isn't documented, then it is not implemented, or there
is no guarantee that it will behave the same way in the next release.

If you want to hang up a connection then you need an smtpd policy
daemon that sends a 421 reply.

        Wietse

> have just found this parameter
> [http://www.postfix.org/postconf.5.html#smtpd_hard_error_limit] for forcing
> server disconnection after some errors. Is it possible to set up similar
> limits, but based on time or number of SMTP transactions?
>
> Thank you,
>
>
> Diego Castillo
>
>
>

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RE: Force SMTP server disconnect

Diego Castillo
>> I would like to force my Postfix SMTP server to disconnect from
>> time to time, in order to prevent its clients from reusing the
>> same connection for too long or for too many transactions.
>
> If you want to hang up a connection then you need an smtpd
> policy daemon that sends a 421 reply.

Would such a policy force Postfix to *close* that particular socket?

I don't want to force a retry at the client side, I want to force
the client to stop reusing the connection and open a new one from
time to time. Ideally after a max number of messages, if not possible
after a TTL for the connection between the SMTP client and Postfix.

I found this other parameter in the documentation:
http://www.postfix.org/postconf.5.html#smtpd_policy_service_max_ttl

My understanding is that this TTL refers to the connection between
smtpd and the policy server, not between smtpd and the SMTP client.

Thank you,


Diego

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Re: Force SMTP server disconnect

Noel Jones-2
Diego Castillo wrote:
>>> I would like to force my Postfix SMTP server to disconnect from
>>> time to time, in order to prevent its clients from reusing the
>>> same connection for too long or for too many transactions.
>> If you want to hang up a connection then you need an smtpd
>> policy daemon that sends a 421 reply.
>
> Would such a policy force Postfix to *close* that particular socket?

Yes.  421 is a "special" reply that signals postfix to close
the connection, and requests the client try again later.

> I don't want to force a retry at the client side, I want to force
> the client to stop reusing the connection and open a new one from
> time to time.

Why in the world would you want to do that???  You can
disconnect, but you can't force the client to open a new
connection.  There is no guarantee the client will reconnect
quickly, or ever.

> Ideally after a max number of messages, if not possible
> after a TTL for the connection between the SMTP client and Postfix.

The postfix anvil service can disconnect a client after too
many messages per time interval (independent of the number of
connections used).  This is intended as basic protection from
a runaway client, not to be used for traffic shaping.
http://www.postfix.org/TUNING_README.html#conn_limit

>
> I found this other parameter in the documentation:
> http://www.postfix.org/postconf.5.html#smtpd_policy_service_max_ttl
>
> My understanding is that this TTL refers to the connection between
> smtpd and the policy server, not between smtpd and the SMTP client.

Unrelated.  Postfix does not have a parameter that limits how
long an SMTP client may remain connected, or how many messages
may be sent over a single connection.

What problem are you trying to solve?

--
Noel Jones
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Re: Force SMTP server disconnect

Wietse Venema
In reply to this post by Diego Castillo
Diego Castillo:
> >> I would like to force my Postfix SMTP server to disconnect from
> >> time to time, in order to prevent its clients from reusing the
> >> same connection for too long or for too many transactions.
> >
> > If you want to hang up a connection then you need an smtpd
> > policy daemon that sends a 421 reply.
>
> Would such a policy force Postfix to *close* that particular socket?

Yes.

> I don't want to force a retry at the client side, I want to force
> the client to stop reusing the connection and open a new one from
> time to time. Ideally after a max number of messages, if not possible
> after a TTL for the connection between the SMTP client and Postfix.

Can you describe the PROBLEM, instead of the SOLUTION (limit
connection reuse)?

        Wietse
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Re: Force SMTP server disconnect

Victor Duchovni
On Mon, Nov 17, 2008 at 11:07:44AM -0500, Wietse Venema wrote:

> > I don't want to force a retry at the client side, I want to force
> > the client to stop reusing the connection and open a new one from
> > time to time. Ideally after a max number of messages, if not possible
> > after a TTL for the connection between the SMTP client and Postfix.
>
> Can you describe the PROBLEM, instead of the SOLUTION (limit
> connection reuse)?

If the right solution is to indeed cause clients to re-establish
connections, the most plausible way to do that would be to issue a
"421" response to the "RSET" command after ".". If the client is a
Postfix system trying to re-use connections, it will discard the cached
connection, and open a new one.

Other MTAs may (will) behave differently, but this is the most robust
approach as responding 421 to "MAIL TO", ...  will be taken as a
transaction failure, and will more likely cause the client to defer mail.

The current implementation of RSET is subject only to the "junk command
limit", which limits the number of non-transaction commands *before*
a transaction, but not the total number for the connection lifetime.

The feature the OP is requesting (if it in fact solves a real problem
and is not the result of poor problem analysis), requires new code to
implement an optional connection-wide limit on "RSET" commands.

Note, this feature used on servers, would defeat the "time-domain"
connection re-use strategy in Postfix clients. Postfix switched
from re-use counters to re-use timers, because counters lead to
"slow-attractor" problems, where a very slow busy server attracts all
the available connections. Servers forcing clients into counted re-use
model risk the same phenomenon unless they are behind load-balancers
that implement a "least-connection" load distribution strategy.

The dynamic behaviour of SMTP under load is rather subtle, naive
anti-abuse strategies can damage the infrastructure by forcing sub-optimal
client-behaviour onto legitimate and abusive systems alike.

--
        Viktor.

Disclaimer: off-list followups get on-list replies or get ignored.
Please do not ignore the "Reply-To" header.

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http://www.postfix.org/lists.html or click the link below:
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RE: Force SMTP server disconnect

Diego Castillo
Noel, Wietse, Victor,

Thanks for your replies. Here goes the description of my problem:

I have a pool of postfix servers receiving SMTP traffic from an upstream MTA
via a load-balancer. Both the MTA and the LB are out of my control; the MTA
may not be a postfix, and the LB may not be using a "least-connection"
strategy.

The LB should balance messages coming from the MTA, but it is actually
balancing new connections only. The MTA is opening very few simultaneous
connections, and reusing each of them for thousands of messages. As a
result, only a subset of my pool is actually working, each of them with one
connection being intensively reused.

In order to improve this situation, I want to close the connection at the
server side from time to time, thus forcing the MTA to open a new connection
for the next message to come. The new connection will be balanced and may be
opened against a new postfix server of the pool, which otherwise may remain
unused for hours.

So my understanding is that I need to launch anvil at each server of the
pool, and make it reply 421 from time to time to its local smptd, so that
this smtpd will disconnect from its client (the MTA). However, there is no
way to force this 421 reply at anvil based on total connection duration or
total number of transactions. The existing parameter that may be close to
what I'm looking for could be
[http://www.postfix.org/postconf.5.html#smtpd_client_message_rate_limit],
which is rate-based.

If I use this parameter, how can I instruct anvil to reply with 421 and not
any other non-special reply?

At what point of the SMTP transaction will the disconnection happen, after
acknowledgement of a DATA command?

The SMTP client may not be sending any RSET command at all, just thousands
of MAIL-RCPT-DATA series...

Regards,


Diego

-----Original message-----
From: [hidden email]
Sent: lunes, 17 de noviembre de 2008 17:29
To: [hidden email]; [hidden email]
Subject: Re: Force SMTP server disconnect

On Mon, Nov 17, 2008 at 11:07:44AM -0500, Wietse Venema wrote:

> > I don't want to force a retry at the client side, I want to force
> > the client to stop reusing the connection and open a new one from
> > time to time. Ideally after a max number of messages, if not possible
> > after a TTL for the connection between the SMTP client and Postfix.
>
> Can you describe the PROBLEM, instead of the SOLUTION (limit
> connection reuse)?

If the right solution is to indeed cause clients to re-establish
connections, the most plausible way to do that would be to issue a
"421" response to the "RSET" command after ".". If the client is a
Postfix system trying to re-use connections, it will discard the cached
connection, and open a new one.

Other MTAs may (will) behave differently, but this is the most robust
approach as responding 421 to "MAIL TO", ...  will be taken as a
transaction failure, and will more likely cause the client to defer mail.

The current implementation of RSET is subject only to the "junk command
limit", which limits the number of non-transaction commands *before*
a transaction, but not the total number for the connection lifetime.

The feature the OP is requesting (if it in fact solves a real problem
and is not the result of poor problem analysis), requires new code to
implement an optional connection-wide limit on "RSET" commands.

Note, this feature used on servers, would defeat the "time-domain"
connection re-use strategy in Postfix clients. Postfix switched
from re-use counters to re-use timers, because counters lead to
"slow-attractor" problems, where a very slow busy server attracts all
the available connections. Servers forcing clients into counted re-use
model risk the same phenomenon unless they are behind load-balancers
that implement a "least-connection" load distribution strategy.

The dynamic behaviour of SMTP under load is rather subtle, naive
anti-abuse strategies can damage the infrastructure by forcing sub-optimal
client-behaviour onto legitimate and abusive systems alike.

--
        Viktor.


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Re: Force SMTP server disconnect

Noel Jones-2
Diego Castillo wrote:

> Noel, Wietse, Victor,
>
> Thanks for your replies. Here goes the description of my problem:
>
> I have a pool of postfix servers receiving SMTP traffic from an upstream MTA
> via a load-balancer. Both the MTA and the LB are out of my control; the MTA
> may not be a postfix, and the LB may not be using a "least-connection"
> strategy.
>
> The LB should balance messages coming from the MTA, but it is actually
> balancing new connections only. The MTA is opening very few simultaneous
> connections, and reusing each of them for thousands of messages. As a
> result, only a subset of my pool is actually working, each of them with one
> connection being intensively reused.
>
> In order to improve this situation, I want to close the connection at the
> server side from time to time, thus forcing the MTA to open a new connection
> for the next message to come. The new connection will be balanced and may be
> opened against a new postfix server of the pool, which otherwise may remain
> unused for hours.
>
> So my understanding is that I need to launch anvil at each server of the
> pool, and make it reply 421 from time to time to its local smptd, so that
> this smtpd will disconnect from its client (the MTA). However, there is no
> way to force this 421 reply at anvil based on total connection duration or
> total number of transactions. The existing parameter that may be close to
> what I'm looking for could be
> [http://www.postfix.org/postconf.5.html#smtpd_client_message_rate_limit],
> which is rate-based.
>

Anvil sounds like the wrong tool for this job - it will defer
all connections from the client until the rate falls below the
level you specify.  If your goal is to force connections to go
to another server in your farm, anvil might work.

> If I use this parameter, how can I instruct anvil to reply with 421 and not
> any other non-special reply?

Anvil always uses the 421 reply and is not adjustable.


--
Noel Jones
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Re: Force SMTP server disconnect

Victor Duchovni
In reply to this post by Diego Castillo
On Mon, Nov 17, 2008 at 05:58:16PM +0100, Diego Castillo wrote:

> Noel, Wietse, Victor,
>
> Thanks for your replies. Here goes the description of my problem:
>
> I have a pool of postfix servers receiving SMTP traffic from an upstream MTA
> via a load-balancer. Both the MTA and the LB are out of my control; the MTA
> may not be a postfix, and the LB may not be using a "least-connection"
> strategy.
>
> The LB should balance messages coming from the MTA, but it is actually
> balancing new connections only. The MTA is opening very few simultaneous
> connections, and reusing each of them for thousands of messages. As a
> result, only a subset of my pool is actually working, each of them with one
> connection being intensively reused.

Define "very few" and "intensively". From what you describe, the sending
MTA has no mail backlog, and is able to deliver mail to you over a small
number of cached connections. This suggests the load is light, in which
case, uneven distribution should not be a problem.

If the upstream MTA were Postfix, it would only cache connections when
the backlog is high enough, and in that case you would typically see
multiple cached connections. Perhaps your upstream is an Exim system,
Exim tries to batch all mail to a site down a single cached connection.

> In order to improve this situation, I want to close the connection at the
> server side from time to time, thus forcing the MTA to open a new connection
> for the next message to come. The new connection will be balanced and may be
> opened against a new postfix server of the pool, which otherwise may remain
> unused for hours.

Is it really necessary to force even distribution? It sounds like you have
sufficient capacity, and the desire for fair loading is somewhat "cosmetic".

> So my understanding is that I need to launch anvil at each server of the
> pool, and make it reply 421 from time to time to its local smptd, so that
> this smtpd will disconnect from its client (the MTA). However, there is no
> way to force this 421 reply at anvil based on total connection duration or
> total number of transactions. The existing parameter that may be close to
> what I'm looking for could be
> [http://www.postfix.org/postconf.5.html#smtpd_client_message_rate_limit],
> which is rate-based.

Anvil will not do what you want, it will respond to "MAIL FROM", not "RSET"
and imposes message rate limits, not per-connection count limits.

> If I use this parameter, how can I instruct anvil to reply with 421 and not
> any other non-special reply?

Anvil always replies with "421".

> At what point of the SMTP transaction will the disconnection happen, after
> acknowledgement of a DATA command?
>
> The SMTP client may not be sending any RSET command at all, just thousands
> of MAIL-RCPT-DATA series...

Are you sure about that? It really should be sending "RSET" to test the
"liveness" of the cached connection. While it may be legal to skip RSET,
it is not wise, and I would hope that your upstream MTAs does not do that.

--
        Viktor.

Disclaimer: off-list followups get on-list replies or get ignored.
Please do not ignore the "Reply-To" header.

To unsubscribe from the postfix-users list, visit
http://www.postfix.org/lists.html or click the link below:
<mailto:[hidden email]?body=unsubscribe%20postfix-users>

If my response solves your problem, the best way to thank me is to not
send an "it worked, thanks" follow-up. If you must respond, please put
"It worked, thanks" in the "Subject" so I can delete these quickly.
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Re: Force SMTP server disconnect

Victor Duchovni
In reply to this post by Noel Jones-2
On Mon, Nov 17, 2008 at 11:06:32AM -0600, Noel Jones wrote:

> Anvil sounds like the wrong tool for this job - it will defer
> all connections from the client until the rate falls below the
> level you specify.  If your goal is to force connections to go
> to another server in your farm, anvil might work.

I think anvil is a poor fit for the problem description. Anvil monitors
plant-wide rates, and may trigger client backoff, while the OP wants
a single connection re-use limit, which does not require an external
counting process, each SMTP server alrwady has all the data it needs
locally.

The OP largely wants the upstream to not cache at all (yielding fair
load). Caching is for situations when the target domain has multiple
MX hosts, some of which may be down (blackhole for SYN), or may perform
expensive connection setup processing. To avoid high delivery latency,
one caches existing connections.

In the OPs case, there may be only 1 target IP (behind the LB) or or
perhaps a few IPs, but all *highly* available. In this case it is far
better to just ask the upstream to disable their connection cache
(though with multiple LB sites, the upstream will be slow if a site
fails completely).

--
        Viktor.

Disclaimer: off-list followups get on-list replies or get ignored.
Please do not ignore the "Reply-To" header.

To unsubscribe from the postfix-users list, visit
http://www.postfix.org/lists.html or click the link below:
<mailto:[hidden email]?body=unsubscribe%20postfix-users>

If my response solves your problem, the best way to thank me is to not
send an "it worked, thanks" follow-up. If you must respond, please put
"It worked, thanks" in the "Subject" so I can delete these quickly.
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Re: Force SMTP server disconnect

Wietse Venema
In reply to this post by Diego Castillo
Diego Castillo:

> Noel, Wietse, Victor,
>
> Thanks for your replies. Here goes the description of my problem:
>
> I have a pool of postfix servers receiving SMTP traffic from an upstream MTA
> via a load-balancer. Both the MTA and the LB are out of my control; the MTA
> may not be a postfix, and the LB may not be using a "least-connection"
> strategy.
>
> The LB should balance messages coming from the MTA, but it is actually
> balancing new connections only. The MTA is opening very few simultaneous
> connections, and reusing each of them for thousands of messages. As a
> result, only a subset of my pool is actually working, each of them with one
> connection being intensively reused.
>
> In order to improve this situation, I want to close the connection at the
> server side from time to time, thus forcing the MTA to open a new connection
> for the next message to come. The new connection will be balanced and may be
> opened against a new postfix server of the pool, which otherwise may remain
> unused for hours.
>
> So my understanding is that I need to launch anvil at each server of the

No anvil is the wrong tool.

> pool, and make it reply 421 from time to time to its local smptd, so that

Use a policy daemon.

        Wietse

> this smtpd will disconnect from its client (the MTA). However, there is no
> way to force this 421 reply at anvil based on total connection duration or
> total number of transactions. The existing parameter that may be close to
> what I'm looking for could be
> [http://www.postfix.org/postconf.5.html#smtpd_client_message_rate_limit],
> which is rate-based.
>
> If I use this parameter, how can I instruct anvil to reply with 421 and not
> any other non-special reply?
>
> At what point of the SMTP transaction will the disconnection happen, after
> acknowledgement of a DATA command?
>
> The SMTP client may not be sending any RSET command at all, just thousands
> of MAIL-RCPT-DATA series...
>
> Regards,
>
>
> Diego
>
> -----Original message-----
> From: [hidden email]
> Sent: lunes, 17 de noviembre de 2008 17:29
> To: [hidden email]; [hidden email]
> Subject: Re: Force SMTP server disconnect
>
> On Mon, Nov 17, 2008 at 11:07:44AM -0500, Wietse Venema wrote:
>
> > > I don't want to force a retry at the client side, I want to force
> > > the client to stop reusing the connection and open a new one from
> > > time to time. Ideally after a max number of messages, if not possible
> > > after a TTL for the connection between the SMTP client and Postfix.
> >
> > Can you describe the PROBLEM, instead of the SOLUTION (limit
> > connection reuse)?
>
> If the right solution is to indeed cause clients to re-establish
> connections, the most plausible way to do that would be to issue a
> "421" response to the "RSET" command after ".". If the client is a
> Postfix system trying to re-use connections, it will discard the cached
> connection, and open a new one.
>
> Other MTAs may (will) behave differently, but this is the most robust
> approach as responding 421 to "MAIL TO", ...  will be taken as a
> transaction failure, and will more likely cause the client to defer mail.
>
> The current implementation of RSET is subject only to the "junk command
> limit", which limits the number of non-transaction commands *before*
> a transaction, but not the total number for the connection lifetime.
>
> The feature the OP is requesting (if it in fact solves a real problem
> and is not the result of poor problem analysis), requires new code to
> implement an optional connection-wide limit on "RSET" commands.
>
> Note, this feature used on servers, would defeat the "time-domain"
> connection re-use strategy in Postfix clients. Postfix switched
> from re-use counters to re-use timers, because counters lead to
> "slow-attractor" problems, where a very slow busy server attracts all
> the available connections. Servers forcing clients into counted re-use
> model risk the same phenomenon unless they are behind load-balancers
> that implement a "least-connection" load distribution strategy.
>
> The dynamic behaviour of SMTP under load is rather subtle, naive
> anti-abuse strategies can damage the infrastructure by forcing sub-optimal
> client-behaviour onto legitimate and abusive systems alike.
>
> --
> Viktor.
>
>
>
>

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RE: Force SMTP server disconnect

Diego Castillo
In reply to this post by Victor Duchovni
> I think anvil is a poor fit for the problem description.
> Anvil monitors plant-wide rates, and may trigger client
> backoff, while the OP wants a single connection re-use
> limit, which does not require an external counting process,
> each SMTP server alrwady has all the data it needs locally.

I guess this means that the single connection re-use limit
could be implemented inside each SMTP server, without any
policy server. But it is not yet available in the current
version of Postfix (I'm using 2.2 myself). Correct?

The upstream is not caching connections because it is not
delivering to multiple hosts. The farm is seen as a unique IP,
highly available, to which all messages must be delivered.

The need to balance is cosmetic now, but I'm looking for a
solution for this issue because, if traffic grows, I'd like
to be able to add servers to my farm, which right now would
be useless.


Diego

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Re: Force SMTP server disconnect

Victor Duchovni
On Mon, Nov 17, 2008 at 07:13:25PM +0100, Diego Castillo wrote:

> > I think anvil is a poor fit for the problem description.
> > Anvil monitors plant-wide rates, and may trigger client
> > backoff, while the OP wants a single connection re-use
> > limit, which does not require an external counting process,
> > each SMTP server alrwady has all the data it needs locally.
>
> I guess this means that the single connection re-use limit
> could be implemented inside each SMTP server, without any
> policy server. But it is not yet available in the current
> version of Postfix (I'm using 2.2 myself). Correct?

Yes, and the same is still true for 2.5 and 2.6 snapshots. A policy
service can force a 421 disconnect, but only at "MAIL FROM", which may
cause the upstream MTA to defer mail or slow down, ...

> The upstream is not caching connections because it is not
> delivering to multiple hosts. The farm is seen as a unique IP,
> highly available, to which all messages must be delivered.

It is then the case that the upstream gets zero value from connection
caching, and really should NOT do so. Can you work with the upstream
admin to disable the connection cache there?

> The need to balance is cosmetic now, but I'm looking for a
> solution for this issue because, if traffic grows, I'd like
> to be able to add servers to my farm, which right now would
> be useless.

As traffic grows, the upstream will need more concurrent connections.
Why do you believe you need a large machine farm to handle this traffic?
Is there a CPU-intensive processing stage? When does it happen
(pre-queue filter or milter? post-queue content_filter? ...)

--
        Viktor.

Disclaimer: off-list followups get on-list replies or get ignored.
Please do not ignore the "Reply-To" header.

To unsubscribe from the postfix-users list, visit
http://www.postfix.org/lists.html or click the link below:
<mailto:[hidden email]?body=unsubscribe%20postfix-users>

If my response solves your problem, the best way to thank me is to not
send an "it worked, thanks" follow-up. If you must respond, please put
"It worked, thanks" in the "Subject" so I can delete these quickly.
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Re: Force SMTP server disconnect

Wietse Venema
In reply to this post by Diego Castillo
Diego Castillo:

> > I think anvil is a poor fit for the problem description.
> > Anvil monitors plant-wide rates, and may trigger client
> > backoff, while the OP wants a single connection re-use
> > limit, which does not require an external counting process,
> > each SMTP server alrwady has all the data it needs locally.
>
> I guess this means that the single connection re-use limit
> could be implemented inside each SMTP server, without any
> policy server. But it is not yet available in the current
> version of Postfix (I'm using 2.2 myself). Correct?

It will not be implemented.

> The upstream is not caching connections because it is not
> delivering to multiple hosts. The farm is seen as a unique IP,
> highly available, to which all messages must be delivered.

You need PARALLEL delivery, not connection reuse limits.

        Wietse
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Re: Force SMTP server disconnect

Victor Duchovni
On Mon, Nov 17, 2008 at 02:06:48PM -0500, Wietse Venema wrote:

> > The upstream is not caching connections because it is not
> > delivering to multiple hosts. The farm is seen as a unique IP,
> > highly available, to which all messages must be delivered.
>
> You need PARALLEL delivery, not connection reuse limits.

Translation to OPs specific situation:

        - Uninstall Exim on the upstream server

        - Install Postfix 2.5.5 on the upstream server.

        - Raise the destination concurrency limit to allow
          use of downstream plant capacity, the default 20
          is likely too low for this environment. A limit
          of 100 or more may be more reasonable.

        - Disable demand caching globally or with transport
          used for this destination.

The place where this delivery channel needs to be tuned is upstream.

--
        Viktor.

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