This may not relate, but the fact that the 2nd & 3rd can't send causes me to ask this. Please click account settings for any account, scroll down left side to Outgoing Server (SMTP). Are there there SMTP servers showing? Do they have different descriptions? If response is 'yes' to both questions, then disregard this concern of mine. If any answer is 'no', then you need either to add additional SMTP servers or to give each a unique description and then verify that each SMTP server is connected to the right account. Thank you.

They all show the same SMTP but different user names with the initial one labeled as DEFAULT. It's been sending mail for the past two days. However, just now when I tried to verify sending, I got a msg saying that the server wasn't accepting my out going msg. from both the primary and a second account.

I changed the secondary account SMTP and was able to send a msg from both that account and the primary account. But, the Outgoing server SMTP settings no longer has the primary account listed but has the secondary account listed twice, once as the DEFAULT. See attached screen shot.

A few days ago I opened Thunderbird on the old computer and tried to set up the accounts exactly as they were there. It was a little difficult because I was using an older version of Thunderbird and the screens weren't the same. It appeared to be working, but just now, no.

change the descriptions so they are all unique. Personally I like to put the email addrss for that user name in the description. It makes the next step much easier.

Now move to the identify for the incoming accounts (above server settings) and check and correct the outgoing server to use. My guess they have all been using the same one.

The error is the message exceeds the size allowed. If you are sending using the wrong SMTP entry, the maximum size for that address on those credential is probably Zero. Fairly typical computer logic.