11/17/22 4PM EST I'm using Tbird 102.5.0 (went back to 102.0.2 & 102.0-64bits = same problem), win 10, xfinity server. Can receive ALL emails. Can send using primary account email but all subaccounts receive but fail to send geting this: Sending of the message failed. An error occurred while sending mail. The mail server responded: Please ensure your email address and authenticating user match when sending messages. Please verify that your email address is correct in your account settings and try again. Just started this morning!?!?!?

Seems like Comcast likes a SMTP name with the email address in it for each of my separate mail boxes. Thunderbird on the other hand wants to use the SAME smtp name for all my Comcast accounts.

   I can send from my 3 Tbird mail boxes but only one at a time having to change the Comcast/Xfinity smtp name as I change Tbird mail boxes -- reading is ok however.
   
   Anyone know how to get around this on Tbird????

I had the same email issue, my secondary accounts would not send, and I've solved it just now 11/19/22 after a useless hour on the phone with Comcast.

You have to add an outgoing server for each of your addresses. This is new to me, I've been at "use default" (ie the "primary") for years. In any case, I carefully added 3 new OUTGOING servers, using "ADD" in the tools/account settings/smtp panel. Outgoing servers appear on the right, when you click "Outgoing Server (SMTP)" at the very bottom of the accounts list, you may have to scroll it to see it.

In the right panel, click "ADD." Each new server should have exactly the same entries as for the default outgoing server (which you can view using "EDIT" for the default server), except that the last item, "User Name" should exactly match each email address name. Carefully check the default settings, write them down for reference as you work this out.

Then, go back and click on each email account name in the list and on the right panel, at the the bottom, the "Outgoing Server" dropdown box. In it will be the new server you just added, choose the one matching each email address. Do each one at a time and test by sending out one email each time because if you screw it up, like I did, it gets confusing.

If you get the error you made a little mistake in spelling etc. First time it works it will ask for the password for that email account so have that ready, and check the box to keep the password in the accounts passwords list. It will now send.

My process comes from this 3 year old Comcast support note which is not as clear as I like:

See: https://support.mozilla.org/en-US/questions/1270491

Titled is: "Error Message-An error occurred while sending mail. The mail server responded: Request failed: Mailbox unavailable-verify address. Address is okay. Help."

Good luck, CK in N.H.