My E2K3SP2 cluster runs like a champ. Mailbox to mailbox delivery in a second or less. But a few times a day inbound delivery of SMTP mail from external domains -- any external domains -- will just halt -- for say, an hour -- or for as little as a few minutes. With 150 mailboxes, there shouldn't be any performance strain, not even a momentary hiccup, and certainly not an hour. A systemic hit, I think, would be seen even in transfers between mboxes on the same server. There is only one virtual server on my cluster. Outbound e-mail is sent to a Proofpoint spam/data loss prevention system -- and occasionally outbound mail SMTP mail will be queued there for fifteen minutes or so. Where should I start analyzing the SMTP service?

hi, According to your description,my suggestion is that you can see the smtp log. See this link first:http://technet.microsoft.com/en-us/library/aa997276(EXCHG.65).aspx hope can help you thanks, CastinLu TechNet Community Support

Thanks, this might help with a rule out of the SMTP service as a culprit. I never see anything in the mailroot\queue directory, during a delay so I'm not thinking it's SMTP. Nonetheless, this will help. Thanks, Bob

What about inbound mail is that filtered via a third party? Maybe you're greylisting\throttling your provider at your gateway. When this occurs and you look at your SMTP logs you're not seeing any entries during this window? If not that your mail server is not getting anything from the external domain, you need to check the next level smtp gateway or your firewall logs.What happens if you restart the smtp service does mail start to flow right away? Also when this occurs next time go to https://www.testexchangeconnectivity.com/ and do an inbound smtp test and see the SMTP transaction says. If it was able to deliver then at least it went past your firewall and from there you need to examine gateways logs. If it didn't the diagnostics will show cause of failure. James Chong MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com

hi, How about your issue now,any update? thanks,CastinLu TechNet Community Support

Sorry for the dealy in replying. It's been determined there is no delay between the spam filter/smart host and the Exchange server. The mail delay is being introduced somewhere outside the mail filter/smart host. I suspect a route in one of our devices is popping up and down; often it's down when a crucial external mail server is trying to send new mail.