Does each account send through a separate smtp server that applies the same user name and password as the sending account, or do both accounts send through the same smtp? Open Tools/Account Settings, select an account in the left pane, then look at Outgoing Server (SMTP) in the lower right pane.

Each account uses the same SMTP server. I spent about 30 minutes on the phone with my ISP and walked through the Account Settings with her. She was totally baffled why one email address allowed me to send out emails and the other did not.

Some mail providers require the sending account (From) to send through an smtp server that has the same user name and password. It appears that both of your accounts use the same server, but it's the one with the user name/password of the account that works. In Tools/Account Settings, select Outgoing Server (SMTP) at the bottom of the left pane, then make sure you have two smtp servers with distinct User Names. Then, assign the correct smtp to the accounts as described above. The first time you send through an smtp that doesn't have the password defined, you'll be prompted to enter it - which should be the same password as the sending account.

WoHoo! I called my ISP back to read them your suggestion. That wasn't the solution but the 2nd tech was able see something fluky in their system that he was able to fix. Somehow my account had been suspended.

Thank you!

This started happening to me too. Out of the blue my secondary email address can receive messages but I can't reply to emails and I can't send a new email either. My main email address works fine. Also spent 30 minutes on the phone with the internet provider help line and was told the account was not blocked or suspended. Both accounts use all the same settings but the username is different. It was all working fine for years until this happened. Sometime around July 21, 2019 was my last successfully sent message.