Have 5 email accounts with Xfinity/Comcast and can no longer send emails from any account

Have you tried clearing your cache and cookies from your laptop?

@innisart 

Cannot send message using the server (#########@comcast.net). Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be sent. (Apple Mail)

I had this problem with a single account previously, and bypassed it using a different outgoing server.  Now all of my Xfinity accounts don't work.  I think this may have to do with the recent security breach and the forced password resetting.  However, I can only access my primary account, and none of the four auxiliary accounts.  But this is a bit of a moot point since not even my primary account can send emails.  This only applies to my laptop;  outbound emails sent from my phone still succeed.

When you log in to the website https://connect.xfinity.com/appsuite are you able to see all of your email accounts?  If not, you need to call Customer Security Assurance.

Customer Security Assurance
Business Hours: 8:00am - 12:00am EST, 7 days a week  
Contact: 1-888-565-4329