Sounds like a nefarious person is abusing your email address as a 'from' and sending out to a load of potential guessed email addresses that do not exist. The server tried to deliver and failed because address does not exist so it could only send a mailor daemon message to whatever email address was in the 'From' field - you. It does not necessarilly mean your account has been compromised. Usually, the person discovers nothing worked and they stop. If you use imap and you do not have anything in 'Sent' to say you sent it, then the scammer/spammer did not access your account to send anything. If you use POP, then logon to webmail and check the 'Sent' folder. If you discover those sent emails then you need to get your password changed immediately whilst you still have access. You may need to contact your ISP - if email address is supplied by ISP - to let them know you are not sending those emails. The only problem would be if spam filters determine your email address is sending spam and decide to put you on their spam list. But this is not Thunderbird causing the problem.