Various possible reasons include:
- The message contained a virus and was rejected. In addition to automatically rejecting the most notorious virus attachments, e.g. .scr and .pif files, we maintain current virus definition signature files on the UNC-Chapel Hill incoming mail servers to filter out viral messages.
- The message was sent from a blocked domain and was rejected. See our Server-Side Spam Filtering document for more details.
- There was a problem on the sender’s e-mail system. This situation should be addressed with the sending party.