milter-reject: END-OF-MESSAGE 5.7.1 Command rejected

[frg62]

Hello,

Yesterday, I launched a new server:
Debian 7.9
Plesk 12.5.30
Postfix + Dovecot + Plesk Milter

And I already have several complaints from users who are not receiving all their mails (on the server).

The senders all mention the same error in the returned mails:
<recipient@myserver>: host myserver[XXX.XXX.XXX.XXX] said: 550 5.7.1
Command rejected (in reply to end of DATA command)

And when I search the maillog, I can indeed find the corresponding error lines, all like:
Oct 27 15:35:51 server postfix/cleanup[32025]: D400781628: milter-reject: END-OF-MESSAGE from sender-smtp[XXX.XXX.XXX.XXX]: 5.7.1 Command rejected; from=<sender-address> to=<recipient@myserver> proto=ESMTP helo=<sender-smtp>

(I can also find other milter-reject lines in the maillog, but with a logical explanation for the reject.)

I have searched the web for an answer,but could not find any that would apply to my case:
- The recipient addresses are NOT full.
- Many other mails sent by the same sender-smtp DO arrive at destination on my server.
- The mails are accepted by MagicSPam before being rejected by Milter.
(I can chek that info in MagicSpam logs)
- Among the refused mails, several are send from Outlook 365 servers (at Microsoft), which makes it even more bizarre.

I cannot see any reason why they should be blocked...

What troubles me, is that the only answer I could find on the Net is:
"Just disable the Milter filtering."

Am I missing something or is it the only option?

 

[frg62]

(At least partially) responding to myself:

After analyzing every rejected mail, I have understood that, even if they are perfectly legitimate, they all have been sent by a SMTP that is NOT defined in the SPF record of the sending domain.

And cherry on the cake, one of my customer uses external spam filtering, provide by MailInBlack, that sends everything to my server, after filtering, via its own SMTP.
Therefore, everything that goes that way, does not respect any SPF rule!!!

That leads me to the conclusion that Milter applies very strict SPF denying rules.
Therefore I need to disable this filtering.

Am I right?

 

[frg62]

OK, When I am wrong, I am wrong!

The whole problem simply was that I had activated the SPF spam protection in the Server-Wide Mail Settings...
I was fooled because apparently, on the previous server, this option, although activated as well, did not work properly.

Conclusion:
Be careful when simultaneously using several spam filters, you could end up not understanding which is doing what.

Not too proud of myself on this one...