Have you contacted you email provider (the company that have their name in your email address after the @)

Thunderbird does not and can not bounce emails. I think there might be an addon that some folk install to allow them to pretend their server bounced the email. But a check of the headers makes the add-ons ruse obvious. So if your emails are truly being bounced it will be an issue with the server. Quite possibly your account is full. It is something that can come to bite folk that use IMAP accounts as all the mail stays on the server until you delete it and in some cases also empty the trash. While some providers allow for terabytes to be used storing your mail, internet service providers rarely are very generousness with their storage space of email. So the space fills quite quickly.

Thank you - much appreciated. It is interesting to know that Thunderbird cannot bounce emails. All but one of my email accounts are POP3 and so mail does not stay around for long. Yes, next step is to talk to my provider as my knowlege of server issues/fixes is nil!

Jane