Hello this can't be a Thunderbird problem - Thunderbird is just an email client, that is, a software that is talking to a mail server that is the mail agent on the Internet. The mail server is the equivalent to a post office - if one sends a mail to it, and it don't know the email address, it's sent back. The mail client has no role in this happening, it's entirely outside its function. This is not a normal occurence, you should check with the mail provider (ISP ?). I have seen this only in the context of an ISP that is buying another and taking as a legacy the mail addresses created with the old ISP, and after some years decide it's not worth anymore to manage these old addresses and so write them off. Normally when it happens the persons having still old mail addresses are forewarned they must take a new account with the new provider's name.