OK, I've seen hopeful solutions to this password problem, which then turned out to be premature.. That may be the case with my "solution", but here goes:
Yesterday a Comcast tech suggested that I go into my Apple Mail preferences window and delete then re-create my email account. I said, "yeah, right, that's too simple to be THE fix for all of us." But, worth a shot anyway. So I did. And so far so good, after about 24 hours - no more password popups - yet.
Here's something interesting (at least in my limited mind). When I recreated the account, it required at least an 8-character password. It would appear that Comcast recently changed their requirement to at least 8 characters. My old password was just 7. So I added a digit. I just wonder how many of you are using a password shorter than 8 characters? Or for that matter, how many have not yet tried recreating the email account.
Another note, the ONLY account that I recreated and lengthened my password on, was my Primary account, not any of my secondary accounts. But I'll be sure to update this message if and when it starts screwing up again.
But again, my password popups instantly went away after I did this and have not reappeared in the last day or so.
I've done everything that everyone suggested and still no success. I have Time Warner Cable and this problem started on the Monday that Wall Street stocks started tanking when the first bailout plan failed. Funny thing happened though is that only 1 account out of my 3 accounts kept asking for my password. Then when Time Warner tech support came over to replace all my cables and cable modem, now the 2 accounts that was working before is now asking me for the password whereas the 1 inactive account is now working fine. Complete opposite effect. No changes were made to Mail settings when this occurred. Hope this is a clue for some of you.
I am doomed:( I've asked this very same question and no call back from Apple and no answer yet on the discussion board.. I don't use comcast I don't know what it is.. so that is not my problem.. I've deleted mail from the applications and reloaded it.. still not working.. keychain repair says there are no problems... I don't use mobileme.. why does my error say mobileme has rejected my password
Also have Verizon acct. and a Sprint phone. If I delete the email acct. from the phone, the Mail pop-ups stop. For now, I do not need email on my phone anyway, and would much rather use my Mac w/o having the pop-ups. I believe it is as stated by another in the group, a conflict where your POP mail is being accessed by 2 different sources - your Mac, and your cell phone.
As far as the link between email on the cell phone and on the Mac, I have stopped the password requests by setting the cell phone (Samsung Instinct) to not automatically get email. Once the phone is set to get new mail manually by hitting 'refresh', the pop-ups in Mail stopped. Our other cell is an LG Lotus, which does not have a manual setting. That one still causes pop-ups, even if Mail on the Mac is set to manually get new mail. So it only works if the phone is set to manually get mail. I believe this is all caused by multiple sources hitting up the server (Verizon, Comcast, etc.) for new mail. Too many request (whether it's from cell phones or multiple computers), and it triggers a request for authentication. Hope this helps!