cannot send email

... Excessive failed authentication ...

This usually means that one or more of your devices is repeatably trying to log in to the mail system, but is supplying an incorrect ID or password. Eventually the system will lock you out for a period of time. The Comcast Security Assurance group (see https://spa.xfinity.com/contact-us) may or may not be willing and able to help you.

If not, you'll have to locate the troublemaker yourself. https://forums.xfinity.com/conversations/email/possible-solution-for-unable-to-connect-to-email-from-apple-maybe-other-devices-after-changing-comcast-password/65a1a8fbf961c6163a367c6f describes one approach.

Greetings, @user_0y93d9! I hope your week has been treating you well. Thanks so much for taking a moment out of your busy day to leave a post on our community forum about your email account. You have definitely come to the right place for assistance.

I tend to agree with @BruceW about the repeated authentication issue. The outgoing mail server is usually the first thing that gets impacted. Please let us know if you are able to clear things up as advised, or if you encounter any issues. Our amazing community is always ready to help.