All of a sudden, Zimbra Network Edition 4.0.1 is experiencing problems sending mail.
What seems to be happening, is instead of postfix looking up the MX record for outgoing mail, it's only looking up the A record and attempting to send to that. Here's an example:
Sep 25 14:46:18 hickory postfix/qmgr[22523]: 86E2D520004: to=<"someone"@msn.com>, relay=none, delay=5, status=deferred (delivery temporarily suspended: connect to msn.com[]: Connection timed out)

If you do a ns lookup for the A record of msn.com, you'll see that it's the as above, but the MX records for msn.com are completely different IPs. This just started all of a sudden, and I can't figure out why.