SMTP server configuration for Apple mail (and related issue)

I use an Imac that was configured years ago to use Comcast for Apple mail. I have 3 Comcast email addresses using POP3 for incoming mail. 3 days ago suddenly (zero configuration changes, upgrades, etc.) my main email address started refusing to send mail. It does receive incoming mail. The other two email addresses are functioning properly.

When the problem email doesn't send a message I get an error that states: "I cannot send message using the server Comcast. The sender address XXXX@comcast.net was rejected by the server smtp.comcast.net. The server response was: Please ensure your mail address and authenticating user match when sending messages". A list of 4 Comcast servers is displayed (Comcast, comcast.net, comcast.net, comcast.net). The message ends up in the Outbox.

The email address works with no problem from webmail. The issue is only when I use Apple mail, which is is what I use all the time.

I'd appreciate some help to try to resolve this out-of-the-blue issue.

1. Does anyone know what could have caused this to happen and what's the meaning of : "Please ensure your mail address and authenticating user match when sending messages"?  What should one do to accomplish this "match"?

2. How can one see and validate the SMTP server's password?

3. Do user names configured for incoming and outgoing servers have to be the same? They were not for years....   I attached a screen capture of what I see.

Thanks in advance.