Do you have each account set to send through a separate smtp.verizon.net (on 465, SSL/TLS) with a User Name (email address) matching the sending account?

https://support.mozilla.org/en-US/questions/1324013

Check that the smtp assignments are correct, and that all incoming and outgoing servers apply an app password. The smtp should use the same app password as the incoming server.

Same settings on both accounts. I get this message when attempting to send from the problem account.

"An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please verify that your email address is correct in your account settings and try again."

But when I check saved passwords there is none for the outgoing smtp server and no opportunity to enter one. I suppose my next move is to remove the account again and re-install.

No need to reinstall the account. The outgoing server will use the same app password as the incoming server if you have the incoming and outgoing servers matched up correctly, i.e. with the same User Names. If you remove the account passwords from Saved Passwords in Options, then restart TB, enter the app password when asked and check the box to have it stored in Saved Passwords.