First check that you can log into each account with your web browser and view e-mails. Then, assuming these are all IMAP accounts and further assuming none of the accounts are att.net or bellsouth.net, you can delete the accounts and re-add them. With IMAP accounts no e-mail will be lost. (Unless the issue is with your ISP, which while not unheard of, is rather rare.) I've experienced where comcast makes a change to something and it effectively "breaks" thunderbird. But when I delete and re-add the account in Thunderbird, everything works fine. Finally, make certain you are using the latest version of TB also. In the past comcast has made changes that a version of TB could not deal with. Then, when I upgraded to the latest, everything worked just fine.