We are running Exchange 2007 and have been doing so for a few years now with only occasional issues. This morning, however, we are not receiving any inbound (external) e-mail. Internally sent e-mail seems to be fine and we can send e-mail out, but nothing is coming in from any domain.
We've rebooted the mail server as well as our domain controller and that seems to have had no effect.
Any ideas about what we should look for to resolve this?
UPDATE: I telnetted into the server and found that the server responds with 452 Insufficient system resources when I perform the MAIL FROM command. We appear to have plenty of disk space and have recently rebooted so memory isn't an issue. Still not working, though.
FURTHER UPDATE: We disabled the Back Pressure threshold as one poster suggested and we are no longer getting the 452 error, but we're still not receiving any e-mails. Perhaps things are backed up at the moment; I'll check again in a little bit to see if things are moving along.