How to stop saved email address from writing from old account

Please clarify what you mean by 'saved email address'? I am having trouble understanding the issue. thank you.

An email address from my Thunderbird Address Book.

But the FROM address comes from the sending account. If that account has not been removed, then that error will happen. I suggest you click 'account setting' from menu and scroll down left sidebar to see if the account is still listed.

I also suggest removing from your addressbook.

As my OP said I was using my current account inbox. The old account is still listed but has been disabled from sending. That is how I realised the "From" address was the old account address.

I was definitely using the correct account inbox, and as I said I repeated the process. Just did it a third time. The repeated times I just went to the address book entry and clicked on Write. The Write window came up with the "From" listed as my old account address at the top. I can see my new account address inbox listed on the Tab at the same time.

If I click on Write at the top of the Thunderbird window the correct address is listed but all addresses in the Address Book are providing the wrong "From" address including a very recent one which was only used with the new address.

Remove what from my address book? The old address? I have now done so. No change.

As long as the old account is still there, the possibility of this continuing is possible. Be sure that it is not set as default. My recommendation is to delete it. If you are keeping it because of old emails, I suggest creating a folder in Local Folders and copying them there so the account can be removed.

I was keeping the old account so I would still get messages from senders that haven't changed to my new address. That is probably not needed anymore. so I can delete the account as you suggest.

As my OP said I was using my current account inbox. The old account is still listed but has been disabled from sending.

No it has not. You can't do that.

I was definitely using the correct account inbox, and as I said I repeated the process. Just did it a third time. The repeated times I just went to the address book entry and clicked on Write. The Write window came up with the "From" listed as my old account address at the top. I can see my new account address inbox listed on the Tab at the same time.

Go to outgoing server (SMTP) is this "disabled account shown there? it must be to be used to send mail and you need to remove it from there.

If I click on Write at the top of the Thunderbird window the correct address is listed but all addresses in the Address Book are providing the wrong "From" address including a very recent one which was only used with the new address.

Thunderbird uses the "default" address from the outgoing server (SMTP) list of account to send mail if it is in any doubt as to which it should use. Have you checked what account you have set as "default"

"No it has not. You can't do that."

Ok, lets just say it is not able to login to the server and I cannot send or receive email from that address. I did something to cause this.

The old address is still set as default. I have not set it as default, it was just once the only address in Thunderbird. I have no idea how to set a default address. I cannot find any way to change this.

Both accounts are listed as Outgoing Server (SMTP)