Just few days ago my customer started to have problem with incoming mails. Sender got info "550 denied by policy"
Logs shows incoming SMTP connections but not shows that received file is processed. I suspect (not confirmed) that at this moment sender is receiving 550 error.
Below is example from log:
13:36:33 984 DMN: MSG 453406 Accepted connection: [xxx.xxx.xxx.xxx] (yyy.domain.pl)
13:36:33 984 DMN: MSG 453406 Receiving file: /media/nss/DATA/ei_waw/wpgate/gwia/third/receive/16c5ccb4.254
13:36:34 984 DMN: MSG 453406 SMTP session ended: [xxx.xxx.xxx.xxx] (yyy.domain.pl)

MSG 453406 is not processed, no Procesing inbound message no Queuing to MTA info for 453406 - looks like this file was put in "blackhole".

This symptom escalates for additional domains ...
Sending hosts are postfix, Exchange others

Sometimes messages are incoming without problems, sometimes as in log.

Any sugestions ??