Issue:
Server has been rebooted. When the server returns, all seems to be okay except that when SMTP servers connect, no messages are ever received.
Assuming you have made no message configuration changes to reject mail (policies, blacklists, etc.), this would turn out to be a networking issue:
1. You may have moved the server from one port to another. The NAT tables are caching the server at the "old" port so communication of message does not actually occur. Reboot or clear out the tables in your routers, switches, and hubs.
2. You may have had a crash and moved a hot drive from one box to another. When you did, you probably had to do a network reconfiguration of drivers unless the hardware is identical. When that was done. Check the following: You forgot to remove the old network adapter before adding the new one so the "bad" one is still active, too. When you added the new one, you used NT which seems to not always take the gateway entry on the first save, so it is missing. You have multiple network ports and have your cables backwards, so the servers make the initial request on one port but the rest of the communication happens on the wrong internal port -- this also usually means you need to update your mail server to listen on all external ports (or the "right" port).
3. You rebooted and another machine has come up on the network and grabbed your static IP (an old backup server that you think is not connnected to the network). The old machine now is intercepting packets meant for your mail server. Get the other machine off the network. Watch for issue #1 above, and reboot the mail server.
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