Effective this pm (11/4/2017), receive message stating email message cannot be sent. here is a sample:
As a test try sending emails to those same people using another non-Comcast account you may have such as Yahoo or Gmail or something like that. Do those emails get through OK? The 550 error series normally means something is wrong with the recipient's email address.
The Comcast/Xfinity email system was down which caused the problem, not the destination address. Thank you for your response, it is good to know that someone was reading my post. The system came back up within a few hours and has not gone down since. I suspect the C/X system was being revamped or upgraded and therefore unavailable to their customers during that time as others posted a similar problem.