AT&T email cannot send Thunderbird mail v78.7.0 in 20.04
Moving from Ubuntu 14.04 to 20.04 Mail settings at the user level are identical for both but only the v52.9.1 can send mail. Also, many of the received emails on 20.04 have no content visible, only the header area. Using synaptic to reinstall Thunderbird made no changes. SMTP server is outbound.att.net 465 There are 3 email addresses being used. On the older v52 machine one of the addresses required a password key which cleared up a receiving problem but cannot send out emails and there is no password shown for the outbound direction only the inbound. The other 2 addresses can send and receive msgs as normal using a standard password and the password manager shows lines for both inbound and outbound. The 20.04 machine throws a send message error "sending of the message failed" instantly. There is no time delay as it goes out and looks for the AT&T server.
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If a password was changed recently or the account was recently added to TB, you would need to use a secure mail key instead of the account password. Accounts with existing passwords would continue to work, until the password was changed.
https://www.att.com/support/article/dsl-high-speed/KM1010523/
Account requiring the long password key was recently combined with another account that acts as the master. Effectively the password change you mention. In hindsight, taking up the offer to combine accounts was a bad mistake. It has cost a lot of time and money not receiving incoming mail from the blocked account for a week.
See this post about sub-account mail keys.