[Exim] Enter Network Password - an Exim issue?

Could Exim be causing this in any way? Any info on what could cause
this? Is there an easy fix?

Issue: Outlook sporadically interrupts you with "Enter Network
Password" pop-up, with username & password correctly filled in.
Eudora does a similar thing, except you have to re-enter the password.

My web host uses Exim 3.36, Red Hat Linux 7.x, Apache 1.3.27. Sorry
cannot be more specific.

Thank you,
S. Walker