This sounds like the same issue as `8503-prtg-smtp-notifications-suddenly-stopped-working`. I didn't see a fix and wasn't sure if I should reply under that question or create a new one. Sorry, if I chose poorly.
I am running; Exchange 2003 fully patched. PRTG 188.8.131.529 on Windows XP fully patched.
SMTP is not happy. It cannot send SMTP notifications.
- PRTG log gives a `Error Sending Email: Connection closed gracefully`.
- If I look at my Exchange server it opens the SMTP connection then times out 10 minutes later and closes the connection.
- Wireshark shows a connection then attempt, PRTG Sends a RSET, the server sends a Resetting, PRTG sends an ACK, then nothing until the server closes connection.
- I brought up a SMTP sensor. The basic SMTP sensor works. If I change it to `Send Mail` it fails like the notifications.
Relaying is allows for this IP on my server and all others who talk to the server (Printers, clients, etc...) don't have an issue.