Unable to receive emails from btinternet.com

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Unable to receive emails from btinternet.com

David Hartley
I am running Postfix on a Synology NAS using DSM 6.2

In general I can receive emails, however I cannot receive emails
from<name>@ btinternet.com.

An example of the sender's failure report is:

Reporting-MTA: dns; sa-prd-fep-040.btmx-prd.synchronoss.net
Arrival-Date: Wed, 27 May 2020 18:31:52 +0100
Received-From-MTA: dns; sa-prd-rgout-001.btmx-prd.synchronoss.net
(10.2.38.4)

Final-Recipient: RFC822; <[hidden email]>
Action: failed
Status: 4.4.7
Remote-MTA: dns; xxxxxxx.uk
Diagnostic-Code: smtp; 250 2.1.5 Ok

The Postfix log sees the BT mail server connect, but does not receive
any data:

2020-06-09T12:04:00+01:00 <myserver> postfix/smtpd[7356]: connect from
mailomta12-sa.btinternet.com[213.120.69.18]
2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: Anonymous TLS
connection established from mailomta12-sa.btinternet.com[213.120.69.18]:
TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: A33018C5A8C:
client=mailomta12-sa.btinternet.com[213.120.69.18]

2020-06-09T12:09:01+01:00 <myserver> postfix/smtpd[7356]: timeout after
DATA (0 bytes) from mailomta12-sa.btinternet.com[213.120.69.18]
2020-06-09T12:09:02+01:00 <myserver> postfix/smtpd[7356]: disconnect
from mailomta12-sa.btinternet.com[213.120.69.18] ehlo=2 starttls=1
mail=1 rcpt=1 data=0/1 commands=5/6

Can anyone suggest what the problem is here please?

Is is a problem at my end, or a problem at btinternet.com?

With thanks

David



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Re: Unable to receive emails from btinternet.com

Dominic Raferd
On Thu, 18 Jun 2020 at 09:46, David Hartley <[hidden email]> wrote:

>
> I am running Postfix on a Synology NAS using DSM 6.2
>
> In general I can receive emails, however I cannot receive emails
> from<name>@ btinternet.com.
>
> An example of the sender's failure report is:
>
> Reporting-MTA: dns; sa-prd-fep-040.btmx-prd.synchronoss.net
> Arrival-Date: Wed, 27 May 2020 18:31:52 +0100
> Received-From-MTA: dns; sa-prd-rgout-001.btmx-prd.synchronoss.net
> (10.2.38.4)
>
> Final-Recipient: RFC822; <[hidden email]>
> Action: failed
> Status: 4.4.7
> Remote-MTA: dns; xxxxxxx.uk
> Diagnostic-Code: smtp; 250 2.1.5 Ok
>
> The Postfix log sees the BT mail server connect, but does not receive
> any data:
>
> 2020-06-09T12:04:00+01:00 <myserver> postfix/smtpd[7356]: connect from
> mailomta12-sa.btinternet.com[213.120.69.18]
> 2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: Anonymous TLS
> connection established from mailomta12-sa.btinternet.com[213.120.69.18]:
> TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
> 2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: A33018C5A8C:
> client=mailomta12-sa.btinternet.com[213.120.69.18]
>
> 2020-06-09T12:09:01+01:00 <myserver> postfix/smtpd[7356]: timeout after
> DATA (0 bytes) from mailomta12-sa.btinternet.com[213.120.69.18]
> 2020-06-09T12:09:02+01:00 <myserver> postfix/smtpd[7356]: disconnect
> from mailomta12-sa.btinternet.com[213.120.69.18] ehlo=2 starttls=1
> mail=1 rcpt=1 data=0/1 commands=5/6
>
> Can anyone suggest what the problem is here please?
>
> Is is a problem at my end, or a problem at btinternet.com?

All incoming emails from BT (mailomta[0-9]+-sa\.btinternet\.com) have
been received by our mailservers ok, the most recent was yesterday

See the accepted answer at
https://serverfault.com/questions/295409/how-to-solve-error-550-4-4-7

I suspect some firewalling action either on your mailserver or on an
intermediate router.
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Re: Unable to receive emails from btinternet.com

@lbutlr
In reply to this post by David Hartley
On 18 Jun 2020, at 02:45, David Hartley <[hidden email]> wrote:
> 2020-06-09T12:04:00+01:00 <myserver> postfix/smtpd[7356]: connect from mailomta12-sa.btinternet.com[213.120.69.18]
> 2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: Anonymous TLS connection established from mailomta12-sa.btinternet.com[213.120.69.18]: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
> 2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: A33018C5A8C: client=mailomta12-sa.btinternet.com[213.120.69.18]
>
> 2020-06-09T12:09:01+01:00 <myserver> postfix/smtpd[7356]: timeout after DATA (0 bytes) from mailomta12-sa.btinternet.com[213.120.69.18]
> 2020-06-09T12:09:02+01:00 <myserver> postfix/smtpd[7356]: disconnect from mailomta12-sa.btinternet.com[213.120.69.18] ehlo=2 starttls=1 mail=1 rcpt=1 data=0/1 commands=5/6
>
> Can anyone suggest what the problem is here please?

Is that really all the log lines? There should be a PERMIT line in there that will include something like this:

Jun 18 05:39:43 mail.covisp.net postfix/smtpd[77265] 49ng2k4wjlz2s14Y: permit: RCPT from camomile.cloud9.net[168.100.1.3]: action=permit for Recipient address=[hidden email] ; from=<[hidden email]> to=<[hidden email]> proto=ESMTP helo=<camomile.cloud9.net>

What is in the helo= field might be a clue, since it looks from the NDR that the mail server is using a private IP address, but without more logging it is hard to be sure. It looks like what you have is a successful connection and the the other side gives up five minutes later, for no reason at all, after sending absolutely no data.

Either bt is expecting something your Mailserver is not doing, or there are other log lines, or their server is ver broken (and the latter seems highly unlikely).

You are receiving emails from other senders, I take it?

(Obfuscating your mail server makes checking things like your DNS records or what your server actually is sending impossible, we cannot assume that sharproad.uk is the domain in question.) If that is the right server, it's definitely weird (it doesn;'t accept ehlo, for one). What version of postfix are you running?

Connected to sharproad.uk.
Escape character is '^]'.
220 sharproad.uk ESMTP Postfix
ehlo mail.covisp.net
502 5.5.2 Error: command not recognized
helo mail.covisp.net
250 sharproad.uk
mail from:[hidden email]
250 2.1.0 Ok
rcpt to:[hidden email]
250 2.1.5 Ok
data
354 End data with <CR><LF>.<CR><LF>
this is a test
.
250 2.0.0 Ok: queued as 52A4F8C1A0A                                                                                                    
quit                                                                                                                                    
221 2.0.0 Bye  

This is what I see on my own server:

Connected to mail.covisp.net.                                                                                                          
Escape character is '^]'.                                                                                                              
220-mail.covisp.net ESTMP -- Please wait                                                                                                
220 mail.covisp.net ESMTP Postfix 3.5.3  
ehlo fake.com
250-mail.covisp.net
250-PIPELINING
250-SIZE 26214400
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250 CHUNKING
mail from:[hidden email]
250 2.1.0 Ok
rcpt to:[hidden email]
250 2.1.5 Ok
data
354 End data with <CR><LF>.<CR><LF>
this is a test
.
250 2.0.0 Ok: queued as 49nh4035Qgz2s14Y
quit
221 2.0.0 Bye

There are some pretty obvious differences here, and the lack of SIZE could be an issue. It is quite possible that btinternet is biting specifically for a SIZE or some other 250 code that you are not sending.

Complete logs for the message and postconf -nf would help solve this




--
When a distinguished but elderly scientist states that something is
        possible, he is almost certainly right. When he states that
        something is impossible, he is probably wrong.


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Re: Unable to receive emails from btinternet.com

Wilfried.Essig@Essignetz.de
In reply to this post by David Hartley
Did you check your certificate?

We had some time ago an issue with one sender, that looked like yours in
the logs. After changing from a self signed certificate to one from
letsencrypt the sender didn't timeout in data anymore. Our certificate
where at that time about two years over end time. :-((

Willi

Am 18.06.20 um 10:45 schrieb David Hartley:

> I am running Postfix on a Synology NAS using DSM 6.2
>
> In general I can receive emails, however I cannot receive emails
> from<name>@ btinternet.com.
>
> An example of the sender's failure report is:
>
> Reporting-MTA: dns; sa-prd-fep-040.btmx-prd.synchronoss.net
> Arrival-Date: Wed, 27 May 2020 18:31:52 +0100
> Received-From-MTA: dns; sa-prd-rgout-001.btmx-prd.synchronoss.net
> (10.2.38.4)
>
> Final-Recipient: RFC822; <[hidden email]>
> Action: failed
> Status: 4.4.7
> Remote-MTA: dns; xxxxxxx.uk
> Diagnostic-Code: smtp; 250 2.1.5 Ok
>
> The Postfix log sees the BT mail server connect, but does not receive
> any data:
>
> 2020-06-09T12:04:00+01:00 <myserver> postfix/smtpd[7356]: connect from
> mailomta12-sa.btinternet.com[213.120.69.18]
> 2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: Anonymous TLS
> connection established from mailomta12-sa.btinternet.com[213.120.69.18]:
> TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
> 2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: A33018C5A8C:
> client=mailomta12-sa.btinternet.com[213.120.69.18]
>
> 2020-06-09T12:09:01+01:00 <myserver> postfix/smtpd[7356]: timeout after
> DATA (0 bytes) from mailomta12-sa.btinternet.com[213.120.69.18]
> 2020-06-09T12:09:02+01:00 <myserver> postfix/smtpd[7356]: disconnect
> from mailomta12-sa.btinternet.com[213.120.69.18] ehlo=2 starttls=1
> mail=1 rcpt=1 data=0/1 commands=5/6
>
> Can anyone suggest what the problem is here please?
>
> Is is a problem at my end, or a problem at btinternet.com?
>
> With thanks
>
> David
>
>
>
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Re: Unable to receive emails from btinternet.com

Viktor Dukhovni
On Thu, Jun 18, 2020 at 04:25:40PM +0200, [hidden email] wrote:

> Did you check your certificate?

That's clearly not the issue.  Random guesses are not helpful.

> > Final-Recipient: RFC822; <[hidden email]>
> > Action: failed
> > Status: 4.4.7

The message expired after multiple retries failed to deliver it.

> > Remote-MTA: dns; xxxxxxx.uk
> > Diagnostic-Code: smtp; 250 2.1.5 Ok

This is only an error if it is a response to initial "DATA" command, for
which the expected response is "354 ...".  Which somewhat suggests a
PIPELINING issue.

> > 2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: Anonymous TLS
> > connection established from mailomta12-sa.btinternet.com[213.120.69.18]:
> > TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
> > 2020-06-09T12:04:01+01:00 <myserver> postfix/smtpd[7356]: A33018C5A8C:
> > client=mailomta12-sa.btinternet.com[213.120.69.18]

After STARTTLS, the client sent "MAIL", "RCPT" and "DATA", with the
connection lost at that point.  Therefore, no handshake issues can
be ruled out.

> > 2020-06-09T12:09:01+01:00 <myserver> postfix/smtpd[7356]: timeout after
> > DATA (0 bytes) from mailomta12-sa.btinternet.com[213.120.69.18]

The connection timed out no data was received for 5 minutes (default
smtpd_timeout).

> > 2020-06-09T12:09:02+01:00 <myserver> postfix/smtpd[7356]: disconnect
> > from mailomta12-sa.btinternet.com[213.120.69.18] ehlo=2 starttls=1
> > mail=1 rcpt=1 data=0/1 commands=5/6

See above.

> > Can anyone suggest what the problem is here please?

    $ posttls-finger sharproad.uk
    posttls-finger: Connected to sharproad.uk[88.97.109.65]:25
    posttls-finger: < 220 sharproad.uk ESMTP Postfix
    posttls-finger: > EHLO amnesiac.example
    posttls-finger: < 250-sharproad.uk
    posttls-finger: < 250-PIPELINING
    posttls-finger: < 250-SIZE 10485760
    posttls-finger: < 250-ETRN
    posttls-finger: < 250-STARTTLS
    posttls-finger: < 250-AUTH PLAIN LOGIN
    posttls-finger: < 250-AUTH=PLAIN LOGIN
    posttls-finger: < 250-ENHANCEDSTATUSCODES
    posttls-finger: < 250-8BITMIME
    posttls-finger: < 250 DSN
    posttls-finger: > STARTTLS
    posttls-finger: < 220 2.0.0 Ready to start TLS
    posttls-finger: sharproad.uk[88.97.109.65]:25: Matched subjectAltName: sharproad.uk
    posttls-finger: [...]
    posttls-finger: Untrusted TLS connection established to sharproad.uk[88.97.109.65]:25: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
    posttls-finger: > EHLO amnesiac.example
    posttls-finger: < 250-sharproad.uk
    posttls-finger: < 250-PIPELINING
    posttls-finger: < 250-SIZE 10485760
    posttls-finger: < 250-ETRN
    posttls-finger: < 250-AUTH PLAIN LOGIN
    posttls-finger: < 250-AUTH=PLAIN LOGIN
    posttls-finger: < 250-ENHANCEDSTATUSCODES
    posttls-finger: < 250-8BITMIME
    posttls-finger: < 250 DSN
    posttls-finger: > QUIT
    posttls-finger: < 221 2.0.0 Bye

ESMTP and STARTTLS work.

> > Is is a problem at my end, or a problem at btinternet.com?

To get more info, try "debug_peer_list = 213.120.69.0/25", and
wait for or solicit more email from BT.

--
    Viktor.

P.S.  Added an explicit "Bcc:" to the OP, to record a delivery in my
      logs, assuming that's the right domain/server, ...
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Re: Unable to receive emails from btinternet.com

David Hartley
A big thank you to all who responded to my request for help.

I can see that I have a lot to learn about SMTP and Postfix, but I will
try to use your suggestions over the weekend.

David

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Re: Unable to receive emails from btinternet.com

Nick Tait
Hi David.

I think I can guess what your problem is, because I had exactly the same
symptom with a different bulk email provider...

Basically this sounds like an MTU issue: The SMTP client
(mailomta12-sa.btinternet.com[213.120.69.18] in your case) is able to
establish the TCP connection to your server, and the first few SMTP
requests and responses work fine, because the packets are small. Then
when it tries to transmit the content of an email (after the DATA
command), the packets get blocked because they exceed the maximum size
that will fit through your Internet connection, and this causes the TCP
connection to stall, and inevitably time out.

The problem I've described above manifests itself due to a combination
of a number of conditions:

1. Your SMTP server has a standard MTU setting.

2. Your connection to your ISP has an MTU smaller than your server's
MTU, and your ISP either isn't performing MSS-Clamping, or it isn't
working properly.

3. The SMTP client connecting to your server is behind a firewall that
blocks ICMP 'Destination Unreachable' packets (type=3) due to 'datagram
is too big and must not be fragmented' (code=4).

In my case:

1. My server was using the default MTU of 1500 bytes.

2. My connection to my ISP uses PPPoE, which adds an 8-byte header onto
all packets travelling between my home to my ISP, effectively reducing
the maximum packet size from 1500 bytes down to 1492 bytes. They did
have MSS clamping set up, but it turned out that it wasn't working for
incoming TCP connections (i.e. from the Internet to me).

3. The sender was blocking ICMP 'Destination Unreachable' packets
(type=3) due to 'datagram is too big and must not be fragmented' (code=4).

I raised this issue with both my ISP and the bulk email provider:

* My ISP confirmed (after performing their own testing) that there was
an issue with their MSS clamping, and that it only worked for outgoing
TCP connections (i.e. from me to the Internet). They offered me a new
home router that could do MSS clamping itself (but I declined because I
wasn't keen on the brand).

* The bulk email provider allowed ICMP 'Destination Unreachable' packets
(type=3) due to 'datagram is too big and must not be fragmented'
(code=4) through their firewall.

However to mitigate any future recurrences (from different SMTP clients)
I also decided to reduce the MTU on my server. There are actually a
couple of ways to do this:

a) The simplest way is to modify your MTU in your network interface
configuration, but exactly how you do this depends on 'what' is managing
your network interface. E.g. Depending on your Linux distribution (e.g.
Ubuntu, etc) you might be using systemd.networkd or Network Manager or
NetPlan?

b) The more complicated way is to set "advmss" option on your default
route in your routing table. (This is actually the way that I did it,
because it only reduces the packet size for traffic that is going
to/from the Internet, not traffic between my servers.)

Hopefully this helps? Feel free to email me directly if you think this
is the cause of your problem, but you need more explanation about
anything above?

Thanks,

Nick.

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Re: Unable to receive emails from btinternet.com

@lbutlr
On 19 Jun 2020, at 02:18, Nick Tait <[hidden email]> wrote:
> 1. My server was using the default MTU of 1500 bytes.
>
> 2. My connection to my ISP uses PPPoE, which adds an 8-byte header onto all packets travelling between my home to my ISP, effectively reducing the maximum packet size from 1500 bytes down to 1492 bytes. They did have MSS clamping set up, but it turned out that it wasn't working for incoming TCP connections (i.e. from the Internet to me).

This would be likely to affect ALL incoming mail, not just mail from BT, wouldn’t it?

(I thought of this, but discarded it because the OP reported only problems with BT)



--
The person on the other side was a young woman. Very obviously a
        young woman. There was no possible way that she could have been
        mistaken for a young man in any language, especially Braille.


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Re: Unable to receive emails from btinternet.com

Nick Tait
On 19/06/20 8:28 pm, @lbutlr wrote:
> On 19 Jun 2020, at 02:18, Nick Tait <[hidden email]> wrote:
>> 1. My server was using the default MTU of 1500 bytes.
>>
>> 2. My connection to my ISP uses PPPoE, which adds an 8-byte header onto all packets travelling between my home to my ISP, effectively reducing the maximum packet size from 1500 bytes down to 1492 bytes. They did have MSS clamping set up, but it turned out that it wasn't working for incoming TCP connections (i.e. from the Internet to me).
> This would be likely to affect ALL incoming mail, not just mail from BT, wouldn’t it?
>
> (I thought of this, but discarded it because the OP reported only problems with BT)

The first 'too big' packet that the SMTP client transmits will get as
far as the ISP, and (because it has the 'DF' flag set in the IP header)
the ISP will discard it and send an ICMP 'Destination Unreachable'
packet back to the sender. If the sender receives the ICMP packet, they
will reduce their packet size (MSS) accordingly, and then retransmit the
smaller packet which will get through. The reduced packet size will also
be used for all subsequent transmissions from the sender on the same TCP
connection.

The problem I described only happens if the ICMP 'Destination
Unreachable' packet gets blocked and doesn't make it back to the SMTP
client.

Nick.

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Re: Unable to receive emails from btinternet.com

Wietse Venema
In reply to this post by Nick Tait
Nick Tait:

> Hi David.
>
> I think I can guess what your problem is, because I had exactly the same
> symptom with a different bulk email provider...
>
> Basically this sounds like an MTU issue: The SMTP client
> (mailomta12-sa.btinternet.com[213.120.69.18] in your case) is able to
> establish the TCP connection to your server, and the first few SMTP
> requests and responses work fine, because the packets are small. Then
> when it tries to transmit the content of an email (after the DATA
> command), the packets get blocked because they exceed the maximum size
> that will fit through your Internet connection, and this causes the TCP
> connection to stall, and inevitably time out.

Postfix users have seen lost connections caused by broken TCP window
scaling implementations (though I am not sure if that would result in
a lost connection after DATA, having received 0 bytes).

If window scaling is the problem, use tcp_windowsize as described
below.

        Wietse

http://www.postfix.org/postconf.5.html#tcp_windowsize

tcp_windowsize (default: 0)
       An  optional  workaround  for  routers  that  break TCP window scaling.
       Specify a value > 0 and < 65536 to enable this feature.   With  Postfix
       TCP  servers  (smtpd(8),  qmqpd(8)), this feature is implemented by the
       Postfix master(8) daemon.

       To change this parameter without stopping Postfix, you  need  to  first
       terminate all Postfix TCP servers:

           # postconf -e master_service_disable=inet
           # postfix reload

       This  immediately  terminates all processes that accept network connec-
       tions.  Next, you enable Postfix TCP servers with the updated  tcp_win-
       dowsize setting:

           # postconf -e tcp_windowsize=65535 master_service_disable=
           # postfix reload

       If  you  skip  these  steps  with  a  running  Postfix system, then the
       tcp_windowsize change will work only for Postfix TCP clients  (smtp(8),
       lmtp(8)).

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Re: Unable to receive emails from btinternet.com

Jan Ceuleers
In reply to this post by Nick Tait
On 19/06/2020 10:18, Nick Tait wrote:

> Hi David.
>
> I think I can guess what your problem is, because I had exactly the
> same symptom with a different bulk email provider...
>
> Basically this sounds like an MTU issue: The SMTP client
> (mailomta12-sa.btinternet.com[213.120.69.18] in your case) is able to
> establish the TCP connection to your server, and the first few SMTP
> requests and responses work fine, because the packets are small. Then
> when it tries to transmit the content of an email (after the DATA
> command), the packets get blocked because they exceed the maximum size
> that will fit through your Internet connection, and this causes the
> TCP connection to stall, and inevitably time out.


If you have administrative control of your broadband router, then you
should increase the MTU of its ISP-facing Ethernet interface by 8 bytes.
Of course this only works if it's capable of jumbo frames but most are
these days.

In my case that's eth0, and this is the interface via which I set up the
PPP connection to the ISP:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1508
        inet 192.168.2.2  netmask 255.255.255.0  broadcast 192.168.2.255
        ether 00:08:a2:0a:89:95  txqueuelen 2  (Ethernet)
        RX packets 5654036  bytes 5241700471 (4.8 GiB)
        RX errors 0  dropped 9031  overruns 0  frame 0
        TX packets 4027451  bytes 1179885101 (1.0 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xdfda0000-dfdbffff 

As you can see I've set its MTU to 1508 bytes such that the PPPoE
overhead drops it back down to 1500, matching the rest of my network.
The PPP configuration also sets the MTU to 1500 for good measure.

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Re: Unable to receive emails from btinternet.com

David Hartley
Hello All,

I am delighted to report that I have found and fixed my problem.

I first followed Viktor's advice and increased the debug level for email
from BT.

This did not reveal much more than we already knew; my server was timing
out waiting for data.

I then followed up the MTU issue suggested by Nick and Jan.

I found that the MTU for my router was 1492, but the MTU for my network
card was 1500.

I followed these instructions
https://support.purevpn.com/how-to-change-mtu-value-on-linux to find the
best MTU value.

My thinking was that this might find additional problems at my ISP or
beyond.

Perhaps not surprisingly the best MTU value turned out to be 1492, so I
set my network card to 1492.

Since then I have received two emails from btinternet.com.

Once again thank you all for your help; I would not have found a
solution without it.

With best wishes.

David

On 19/06/2020 15:14, Jan Ceuleers wrote:

> On 19/06/2020 10:18, Nick Tait wrote:
>> Hi David.
>>
>> I think I can guess what your problem is, because I had exactly the
>> same symptom with a different bulk email provider...
>>
>> Basically this sounds like an MTU issue: The SMTP client
>> (mailomta12-sa.btinternet.com[213.120.69.18] in your case) is able to
>> establish the TCP connection to your server, and the first few SMTP
>> requests and responses work fine, because the packets are small. Then
>> when it tries to transmit the content of an email (after the DATA
>> command), the packets get blocked because they exceed the maximum size
>> that will fit through your Internet connection, and this causes the
>> TCP connection to stall, and inevitably time out.
>
> If you have administrative control of your broadband router, then you
> should increase the MTU of its ISP-facing Ethernet interface by 8 bytes.
> Of course this only works if it's capable of jumbo frames but most are
> these days.
>
> In my case that's eth0, and this is the interface via which I set up the
> PPP connection to the ISP:
>
> eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1508
>          inet 192.168.2.2  netmask 255.255.255.0  broadcast 192.168.2.255
>          ether 00:08:a2:0a:89:95  txqueuelen 2  (Ethernet)
>          RX packets 5654036  bytes 5241700471 (4.8 GiB)
>          RX errors 0  dropped 9031  overruns 0  frame 0
>          TX packets 4027451  bytes 1179885101 (1.0 GiB)
>          TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
>          device memory 0xdfda0000-dfdbffff
>
> As you can see I've set its MTU to 1508 bytes such that the PPPoE
> overhead drops it back down to 1500, matching the rest of my network.
> The PPP configuration also sets the MTU to 1500 for good measure.
>

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Re: Unable to receive emails from btinternet.com

@lbutlr
On 21 Jun 2020, at 07:32, David Hartley <[hidden email]> wrote:
> erhaps not surprisingly the best MTU value turned out to be 1492, so I set my network card to 1492.
>
> Since then I have received two emails from btinternet.com.

But this problem was isolated to only mails for BT? You would get mails fine from, say, google.com and this list?

If so, I am surprised.


--
The only good thing ever to come out of religion was the music.


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Re: Unable to receive emails from btinternet.com

Matus UHLAR - fantomas
>On 21 Jun 2020, at 07:32, David Hartley <[hidden email]> wrote:
>> erhaps not surprisingly the best MTU value turned out to be 1492, so I set my network card to 1492.
>>
>> Since then I have received two emails from btinternet.com.

On 21.06.20 09:59, @lbutlr wrote:
>But this problem was isolated to only mails for BT? You would get mails fine from, say, google.com and this list?
>
>If so, I am surprised.

I guess there are not so many networks dropping ICMP packets.
IIRC this was reported for yahoo but that was years ago.

It apparently comes from lack of knowledge and stupid idea that ICMP is bad
and may be blocked safely.



--
Matus UHLAR - fantomas, [hidden email] ; http://www.fantomas.sk/
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Varovanie: na tuto adresu chcem NEDOSTAVAT akukolvek reklamnu postu.
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