I have seen a load of these since the recent changes at Spectrum have highlighted incorrectly configured Thunderbird accounts with that provider, but I will not guess. Who is your mail provider?

ISP is comcast. one email account is a 'forwarded " acct from another provider to comcast, but I never had an issue with that before.

Probably two issues here: first, you must have separate smtp servers for each account, and point each account to the correct smtp server:

https://support.mozilla.org/en-US/questions/1350593

Second, Comcast accounts now use OAuth2 authentication instead of 'normal password' in Account Settings for the incoming and outgoing.

Well, I believed I followed your instructions as outlined and here's the message I received from TBird when trying to send a test message. I also have a mail washer from Firetrust and I also changed the info for the outgoing and incoming mail to reflect what is in TBird. "Account 1" has smtp1.comcast.net...the remaining 2 accounts are numbered accordingly and pointed to their respective email account names Where did I go wrong?

I think all the Comcast smtp should have the same settings except for the User Name (email address) and the Description, which can be smtp1, smtp2,..., to make it easier to point an account to the correct smtp.

Server Name smtp.comcast.net, port 587 (STARTTLS) or 465 (SSL/TLS), authentication = OAuth2, User Name = email address.

https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email

So after many gyrations of emails/passwords/server changes, here's what I'm experiencing: I have 3 email accts. Let's call the Email1, Email2, and Emal3. As the settings exist now, I am able to send test emails between emails 1&2 and send emails to email 3 from both 1&2. However, I am not able to send any emails to accts 1 or 2 from email 3. Email1 & 2 are on the same smtp server but I had to remove the smtp1 and 2 designations in order for them to work. Email 3 server keeps switching to email 1 server and the server won't authenticate the user name or password. It's odd as both email 1 & 3 are on xfinity and email 2 is a sub acct of email 1, being hosted by another ISP. I attempted to use OAuth for the email accts, but the server wouldn't accept it, so I went back to the normal passwords and it was happy and worked to the extent I have described above. Bottom line is that email 1&2 work just fine, but email 3 can receive but not send without reconfiguring the outgoing server manually. One note- my smtp edit button pulls down the menu but it has one server designated as the "default" server. I don't see anything to remove that designation. The Mailwasher prgm is easy to configure and seems to be working fine with Tbird to the extent I've described. Something is screwed up with the outgoing server settings on email 3. Maybe contact Xfinity? Thoughts?

Are these all Comcast accounts? Note that OAuth2 is not supported for all providers, even if the option appears in settings. Perhaps check that 3rd-party access is enabled for Comcast accounts.

The Default smtp only means it is defined for the Default account, which is the one at the top of the left pane of Account Settings.

Two of the accts have a comcast domain (accts 1&3) Acct 2 is being directed to comcast by another ISP (long story). However Acct 2 is working fine, it's the comcast accts that aren't accepting the passwords. I'm beginning to believe that the issue is with comcast as my wife's account is also having issues getting incoming mail. I now have all accts set to port 995 and security to SSL/TLS including my spam program, Mailwasher. Only acct 2 is verifying the connection with the server. Accts 1&3 are not. I have double checked the passwords and incoming and outgoing settings. It's now letting messages through intermittently. I had this issue before and it was on comcast's end. It seems when it gets over 90F here in FL, comcast has issues, specifically with their email system. My next call is to them. I'll let you know how this turns out! Stay tuned....LOL

Are you entering passwords in the normal TB prompt or a browser-like window? It should be the latter if the authentication method is OAuth2.