Is Gmail involved here? For instance, if the account you deleted was provided by Gmail, it's possible that a remaining account was and still is using the Gmail smtp server to send. When it is configured like this, Gmail silently replaces the sender's address with the Gmail address. Note that removing an account in Thunderbird eliminates the incoming (pop or imap) part, but leaves the outgoing (smtp) part in place. It's possible but unlikely that other email providers may do the same. Some would process your message without comment, and without changing the "from" address. Others might point-blank refuse to process a message sent with a different "from" domain. Open the settings for your preferred account(s) and check which smtp server has been selected. IIRC it is under Server Settings.