QUOTE(Cyrilla @ Jul 29 2010, 10:33 AM)

Ah, Solari - you are a Pooter Whizz, I forgot!
Umm...how long have you got???
About 10 days ago I started getting a window popping up on Outlook Express asking me to put in my user name and password. I tried it several times and it just kept popping up again and again. During the call to Calcutta I tried putting in my husband's user name and password (we use the same machine) and it cleared and my mails came through. It was fine for a few days but then it started happening again. Sometimes it's OK and sometimes not but it's getting worse. I tried putting in OH's details again and now I've started getting some of his e-mails! When I've forwarded them to him they get sent back to me ARGH!!!
I'm able to access my mails through the VirginMedia website but WWWWWAAAAAAAAAAAAAAAHHHHHHH this is driving me nuts!
Help....

If it works with one account but not the other, then it looks like some sort of misconfiguration either your side or theirs. You'll have different accounts defined for both of your mailboxes. I'd wager that perhaps on your account settings, there's a box unticked or ticked where it shouldn't be (something like an authentication setting for example). The fact that you can log in via the webmail system suggests that your password doesn't need a reset.
Not at home so can't really fire it up in a VM, but in Tools > Accounts, you can bring up each account > edit/properties and view all of the settings - I'd take a note of all boxes that are ticked and the settings in each text box for the other account that works, then compare it with your own.
Do you use seperate logins for the same machine btw?