I've had a problem after changing from dialup to different broadband - but keeping my old email addresses.

The new ISP doesn't like it and it stopped emails .. even though they were installed as new accounts.

Apparantly, it can be cured by clicking on tools/accounts/mail, highlighting the account with a problem. Open the servers tab and make sure the box at the bottom labelled "server requires authentication" is ticked.

It took me 2 hours .. but job done.