[Exim] POP3 server re-prompts weekly for passwords

Our users are complaining that EXIM keeps triggering the re-prompting of
POP3 password entry within their email applications about once per week.
The error that EXIM generates to their email application is "ERR Sorry
cannot find an owner for the domain (theirdomain.com)". I have no idea
what this means. The users simply type the same password they had stored
and it logs right in.

This is very annoying. Anyone have any ideas?