Today we experienced a problem with our Help Desk product in conjunction with using e-mail. We started getting rejected e-mail messages from the Help Desk product that contained a message at the bottom that said "E-mail inbox POP3 protocol failure" (followed by some random text).
When submitting an e-mail to the Help Desk Ticket Submission e-mail address, a new incident was not being created in the Help Desk product.
Furthermore, e-mail notification for manually created incidents were not getting sent out.
After digging around we found that the mailbox contained an e-mail message (a real Help Desk request) that had a 399MB file attachment. The e-mail account has a 250MB quota. We deleted the email message with the large attachment and after a minute or so, notifications started working again and we were able to create new incidents via email.