1. When you create an email account you normally choose your own password for it. If you have set up several accounts and chosen the same password for them all, and you now want a different one for each, then you need to go to the companies that provide the accounts and choose new passwords. Usually you can do this via the providers' websites. When you have set up new passwords with your email providers, you then need to change the passwords for those accounts in Thunderbird. Here is how to do it: http://kb.mozillazine.org/Setting_and_changing_email_passwords 2. When you say 'a bounce back notification', if you mean an error message like 'this email could not be delivered to [email protected]', then that message has been generated by the comcast server, not by Thunderbird. You need to talk to Comcast to find out why their server is sending it erroneously when in fact the message is reaching your Inbox successfully.