EVENT # 2048076
EVENT LOG Application
EVENT TYPE Warning
EVENT ID 12015
DATE / TIME 5/19/2012 10:57:55 AM
MESSAGE An internal transport certificate expired. Thumbprint:98F05ABC27FEB37D5C156FD7E9FFBFFC0B098288
I had a 3 year UCC Exchange certificate on this server that IIS, POP, IMAP, and SMTP were using. But for some reason, this "internal transport certificate" was using a self signed certificate. It seems as though the solution was as simple as running "New-ExchangeCertificate" from the Exchange Shell. I had already run this to replace my internal certificate, but here's what it looks like when you run the command, it prompts you to overwrite the SMTP certificate for internal processing (noting that the 3rd part SSL cert takes precedence for external communications).