Error code 554 might be that the message is possibly spam or that the content is somehow violating comcast policies. You might compare content of these messages with content of other messages going to comcast users. This is not a thunderbird issue. Good luck.

I have two Comcast addresses I send to, my brother and sister-in-law. I do not send spam messages or any kind of message that could violate Comcast policies. I stated this happens sporadically. 4 other messages were sent to my brother today and all went through. Today's message was about a hint for doing laundry. It was also sent to AOL and HotMail addresses of family members and went through. Many times the EM is my Wordle puzzle result. Would appreciate advice on how my relatives could communicate the issue to Comcast--phone number / help page / ??

You might try some web searches for error 554, especially for comcast. Sometimes, the link in error message gives more specific information. I am no guru in error messages, and 554 can be a variety of stuff. There may be others on the forum with more specific ideas. Doing a quick search for comcast error 554 a number of web links popped up.