If you have sent to it then it is probably in your address book. Have a look in the Collected Addressees address book. Thunderbird puts addresses in here when you send to them, if they are not already elsewhere in the Address Book.

Another way to go about this is to address a message to the troublesome address, then save this as a draft. In the Drafts folder you can view the message, and in its header pane you can select the addressee and edit his Contact card.

acullingham said

I entered my husband's email address with an error in it a long time ago (it's not entered in the address book, it's just a memory) and now sometimes I still send emails to this non-existent address. It's causing me problems! How can I remove ithis non-existent email address from the recipient list?

Don't know why I hadn't seen it in the address book before. Thank you Zenos!!!!!