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List:       postfix-users
Subject:    Re: Refuse mail from hosts with closed port 25
From:       Jim Reid <jim () rfc1035 ! com>
Date:       2019-09-16 13:03:50
Message-ID: 52A30861-A83D-4C1B-8243-301FF7957C70 () rfc1035 ! com
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> On 16 Sep 2019, at 13:47, Paul van der Vlis <paul@vandervlis.nl> wrote:
> 
> How can I refuse mail from hosts who don't have an open port 25?
> 
> What do you think from such a check?

It's a stunningly bad idea. Don't do it.

Many enterprises and cloud-based mail providers have discrete \
servers/systems handling inbound and outbound mail. In these setups, the \
servers sending you email won't have a listener on port 25 -- or any other \
port -- for inbound email.

> Is there more needed?  E.g. a list of exceptions for some big providers?

It'll be impractical to maintain a workable whitelist. There will probably \
be too many false positives and negatives. And the approach probably won't \
be an effective anti-spam measure either. But if you want to try the \
experiment and report back, go ahead. It'll only be you and your customers \
who will have to deal with the consequences.


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