Error 0x7d6 occurred while rendering message 0008-0000008949b3 for download for user firstname.lastname@example.org.
According to Microsoft, this error occurs when email are being retrieved from the Exchange server both through POP3 and Exchange services directly. See the additional links for details.
In our case it was because the users were retrieving emails both through Outlook and a software called Goldmine (which utilizes POP3) at the same time.
You would either need to close one of them, or exclude this error event from the event log.