From postfix-users  Tue Sep 17 00:16:38 2019
From: Chris Wedgwood <cw () f00f ! org>
Date: Tue, 17 Sep 2019 00:16:38 +0000
To: postfix-users
Subject: Re: Refuse mail from hosts with closed port 25
Message-Id: <20190917001638.GB30303 () aether ! stupidest ! org>
X-MARC-Message: https://marc.info/?l=postfix-users&m=156867988727587

> How can I refuse mail from hosts who don't have an open port 25?
>
> What do you think from such a check?

i have tried this, it's not useful, so i didn't leave the check in
place

it's very common, perhaps even the norm that the IP address which
delivers mail to me itself will not accept an incoming port 25
connection

> I've investigated why somebody did not receive mail from a virtual
> machine, and I found out her provider (reviced.nl) refuses all mail
> from a host what does not have port 25 open. I have much problems
> with spam and I would like to reduce it.

this will stop a lot of legitimate mail

and probably not stop much spam