Recently users have been reporting that mail randomly is not delivered from outside SMTP servers to internal accounts.
What's strange with an issue I've tried to track down today, I can see Google's servers connection all the way through to the Postfix/Cleanup process. After this, from what I understand, it should pass off to the Postfix/Qmgr where the mail is delivered (this is probably horribly over simplified)
Sep 3 14:32:17 lamphost2 postfix/smtpd: connect from mail-pw0-f45.google.com[126.96.36.199] Sep 3 14:32:17 lamphost2 postgrey: action=pass, reason=client whitelist, client_name=mail-pw0-f45.google.com, client_address=188.8.131.52, sender=****, recipient=**** Sep 3 14:32:17 lamphost2 postfix/smtpd: 9E5718F0C28C: client=mail-pw0-f45.google.com[184.108.40.206] Sep 3 14:32:17 lamphost2 postfix/cleanup: 9E5718F0C28C: message-id=<4C7EF5F8.4050500@****>
That's the end of the whole process, nothing more. The message simply falls off the radar and the user who originally sent the message is notified that the message was deferred or it timed out, etc. It's very random, though, as stated, I've had more people who are all using different SMTP servers to send to our system that have had issues.
All that has been done thus far was increasing the smtp_connect_timeout in Postfix from the default (which I think it 30s) to 35s. Nothing else has been changed on the system.