I'm running Exchange Server 5.5 Service Pack 4 (SP4) on Windows NT Server 4.0. Sometimes, it takes a couple of days to receive inbound messages from some external users, and sometimes I don't receive them at all. My ISP couldn't find any problem with my routers. I built another server on a totally new box, but I'm still experiencing the same problem. What could be causing this difficulty?
Mail delays might have nothing to do with your messaging system. The first thing I'd investigate is whether any patterns exist among messages that arrive late (or not at all). If you notice that all the messages originate from a small number of domains, verify that you haven't restricted inbound mail from those domains. (Although if mail is just delayed and not blocked, relay restrictions won't be the cause.) Next, turn on message tracking for all servers between your firewall and a recipient, then ask the sender to send a message that you can track. When the message makes it to your gateway server, you can use the message-tracking logs to follow its path to the recipient. The logs will tell you whether the delay occurs before the message reaches your Internet Mail Service (IMS) or after the IMS has accepted the message for the Message Transfer Agent (MTA) to route. You might also consider monitoring your ISP's connectivity by sending messages at predictable intervals, then observing whether the messages arrive on time.