ISP open relay

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ISP open relay

Wesley Peng-10
Hello

My ISP email even doesn’t require SMTP AUTH. Will they be acting as open relay? How to stop abuse of outgoing mail?

Regards 
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Re: ISP open relay

Dan Mahoney (Gushi)
Presumably they know what your IP is because they gave it to you. That’s the authorization.

I am willing to bet that if you tried to send mail from off network you wouldn’t be able to without doing SMTP auth

Sent from my iPhone

> On Jan 12, 2020, at 16:15, Wesley Peng <[hidden email]> wrote:
>
> 
> Hello
>
> My ISP email even doesn’t require SMTP AUTH. Will they be acting as open relay? How to stop abuse of outgoing mail?
>
> Regards

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Re: ISP open relay

Gerben Wierda
Some ISP’s even go further my catching all traffic to port 25 to any system outsuide their network (other than to their own MTAs) blocking it or directing that to their own MTA. That is because of course one hacked system in their network means an open relay from their network and that is bad for the reputation of the network. So, they may force all clients to go to their relay. I guess not many ISPs still do this anymore, though some may still block port 25 to anywhere but their own mail relay.

It is many years ago, but at some point my mail could not be delivered (I was sending to 25 with STARTTLS) and after investigation it turned out that the ISP (in this case telecom provider KPN) was redirecting all traffic to their own MTA which then spoofed being my mail server. It was very funny to try connect to my mail.rna.nl postfix MTA to be greeted with “This is sendmail at mail.rna.nl”. Of course this went wrong as soon as authentication was started.


On 13 Jan 2020, at 01:38, Dan Mahoney <[hidden email]> wrote:

Presumably they know what your IP is because they gave it to you. That’s the authorization.

I am willing to bet that if you tried to send mail from off network you wouldn’t be able to without doing SMTP auth

Sent from my iPhone

On Jan 12, 2020, at 16:15, Wesley Peng <[hidden email]> wrote:


Hello

My ISP email even doesn’t require SMTP AUTH. Will they be acting as open relay? How to stop abuse of outgoing mail?

Regards


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Re: ISP open relay

Jaroslaw Rafa
Dnia 13.01.2020 o godz. 15:50:59 Gerben Wierda pisze:
> Some ISP’s even go further my catching all traffic to port 25 to any
> system outsuide their network (other than to their own MTAs) blocking it
> or directing that to their own MTA. That is because of course one hacked
> system in their network means an open relay from their network and that is
> bad for the reputation of the network. So, they may force all clients to
> go to their relay. I guess not many ISPs still do this anymore, though
> some may still block port 25 to anywhere but their own mail relay.

I once found an ISP that blocked port 25 completely, even not allowing to
connect to their mail relay (actually, they didn't provide any mail relay at
all to their users, so there was nowhere to connect to). You were forced to
use ports 587 or 465 for outgoing mail.
--
Regards,
   Jaroslaw Rafa
   [hidden email]
--
"In a million years, when kids go to school, they're gonna know: once there
was a Hushpuppy, and she lived with her daddy in the Bathtub."
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Re: ISP open relay

@lbutlr
On 13 Jan 2020, at 07:58, Jaroslaw Rafa <[hidden email]> wrote:
> You were forced to use ports 587 or 465 for outgoing mail.

Yes, that is a sensible ISP.



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