Best you don't !
To clarify, reading POP3 is 1 thing, but if they need to send too, and you let them use the SMTP service, that part could be dangerous.
You can protect it, but the relay issue is always a minefield.
Marc
If 'something' 'somewhere' gives 'some' error, expect random guesses or no replies at all. Please specify details.
Free Tip: The F1 Key does NOT destroy your PC!