cannot send email

When I was trying to send email I was getting a message that my address isn't recognized by the system and that I needed to make sure it matched the authentication.  So I went into setting on Mac Mail and saw that for both my husband and my emails. we are using the same SMTP setting with his email address.  When I tried to change the SMTP settings to my email I got a message that only a single SMTP account is allowed which is probably why they were both using the same setting (but Comcast allows multiple emails so this is confusing).  We've had both Comcast emails and the same settings for years so why has this suddenly started??  I can sent emails without issue from his address and also when I go into my Comcast email online.  

Thank you so much for this context, user_jr7fp4. A few additional follow-on questions from this- "I can send emails without issue from his email address." Is this set up also through MacMail, or another third-party email client? When accessing via the Webmail site, is the Turn your access to third-party email programs on or off in Xfinity Email enabled?