Most probably it is a combination of your server's Greylisting, which temporary rejects all new messages for approx. 10 minutes. The sending server - if configured corectly - should resend the same message within retry schedule, which is something like few times every 5 minutes, then every 30 minutes, and if not success, retry every hour or two for next 3 days.
So, it must be sending server having either some not recommended retry schedule setup, or it is somehow incompatible with greylisting (whatever that would mean).
Is your issue each time from the same sending server? Or is it general 2 hours delay for all mail, regardless of origin? |