>>>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?

>On 16 Sep 2019, at 9:17, Kevin A. McGrail wrote:
>>Paul, I wrote a module which I need to update on Perl's CPAN called
>>Net::validMX that we use to reject IPv4 domains that aren't properly
>>setup to receive mail from sending to us.  We've used it in 
>>production
>>with MIMEDefang.  And as a small, boutique ESP for over a decade, 
>>likely
>>closer to 15 years with no complaints/FPs of note.

On 16.09.19 09:59, Bill Cole wrote:
>I don't believe that Net::validMX does anything more *at the domain 
>level* than Postfix's built-in reject_unknown_sender_domain 
>restriction. Its check_email_validity() may be a bit more strict than 
>Postfix's built-in address sanity checks.

you can both still add checking for bogus MX records, e.g. poinging to
private/reserved address space, MX pointing to CNAME etc.

The first can be done in postfix by using check_sender_mx_access, for the
latter you need using something like rfc-clueless blacklist.

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