[pfx] Per recipient sender access via an access policy delegation server

Hi

I'm posting this message in case anyone would find it useful to have an 
access policy delegation server that implements per recipient sender access.

I have been using check_sender_access as a deny list for senders that I 
want to block from sending me email, usually emails that started off as 
legitimate but after unsubscribing still keep coming. The issue is that 
when I block a sender, I do so for any recipients that use the server, 
not just me. While in practice that hasn't caused many issues, it always 
worries me that I might block email other users want to receive.

So I had it in mind to write an access policy delegation server that 
implements  per recipient sender access. I believe that it's not 
possible to achieve this within Postfix configuration directly and that 
the policy delegation protocol was developed just for such use cases. 
With a little bit of help from AI I have got a working prototype in 
python that queries a mariadb table. It's a single long running process 
that can handle multiple incoming connections. The lookup 
functionality/return codes work similar to access maps, but has the 
additional key of the envelope recipient address.

I realize that probably other policy servers contain similar 
functionality bundled with other capabilities, but I decided to do 
something specific for this single need. At the moment I did this for my 
own use, but if anyone has a similar need I'd be happy to publish the 
code, if contacted on or off list.

John






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