email won't send

Thanks for responding. Yes, access to third-party email programs is enabled. 

Sorry to learn your email concerns have returned @user_z34mz9.Can you go through these settings to make sure your 3rd party clients are set up correctly. To set up third-party email clients for Xfinity Email, first, allow access to third-party programs on the Xfinity Email website. Then, select your email program type (e.g., Outlook, Apple Mail, etc.) for specific setup instructions. For general settings, use the following: Incoming Mail Server: imap.comcast.net (Port 993 with SSL ON), Outgoing Mail Server: smtp.comcast.net (Port 587 or 465 with TLS/SSL). Ensure you have your Xfinity ID and password ready for sending messages. For detailed steps, refer to the specific client instructions provided by Xfinity.

For more details see: https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email

So yes, access to third-party programs is indeed checked in the security settings, as it has been. The setup instructions are just that: how to set up or remove third-party programs. But I don't want to set up or remove programs; I want the third-party programs which have been working for many years to continue working. The link does not appear to address this issue. So I don't know how to proceed.

user_z34mz9 you bring up a great point, but I believe our information was to address that very underlying issue. Have you attempted to toggle off and back on the 3rd party option and then redone the setup instruction from scratch?

I did toggle off and on the 3rd-party option, and it seemed to make no difference to the behavior of my email. I did not do the setup instructions from scratch, because my account is already set up. In the meantime, my email appears to have begun working properly again (for now). The sudden appearance and disappearance of the problem, unrelated to any actions on my part, leads me to believe that the cause of the problem is not on my end.