I don't know what POP3/ISP you're using, but I was having a similar issue
until Verizon installed a rack of junk on my pc during a service call. This
is what I found out:
1) Had to delete all the non-essential Verizon stuff (sped up my pc like
gangbusters!), especially their Help and Service center.
2) They installed an additional email account to my Outlook that I didn't
need (was having issues connecting between my Yahoo! email account outgoing
SMTP settings; got that fixed by using Verizon's outgoing SMTP address, and
all was well).
3) Because all that was done, under Outlook's Tools/Email
Accounts/Change/Advanced settings, the Remember Password block was UNCHECKED.
I checked it, then tested settings, and everything was fine. No more popups.
So, if you're still having the issue, check your email settings and make
sure your Remember Password block is checked (be sure the password is typed
in the space under your email address). That should fix it. If not, then
.... well ... I don't know what else you could do. I've tried deciphering all
the technical jargon some of the MVPs mentioned about changing registry
entries, etc., but it's just too much for me. If you've tried it and it
worked for you, then that's a good thing!
Peace,
Vi