Diagnostic-Code: smtp; 550 5.1.1 <firstname.lastname@example.org> User doesn't exist
A lot of people (but not all!) trying to send email to me get a bounce message that I don't exist. I can send email from email@example.com to firstname.lastname@example.org and it will bounce! I can send mail to other email fine. I AM getting email (my more or less normal load of junk mail), and this problem persists on Chrome browser, IMAP email, Outlook & etc. Comcast support hasn't been helpful. It seems that this new email has lots of problems. I see that other people have in the past had a similar problem. I reset my password, no luck. What's the magic juice?
Comcast Security Assurance should be able to help you with this. Give them a call at 1-888-565-4329 (from http://constantguard.comcast.net/help/report-abuse/). Note that this is NOT one of the general customer service numbers.
The first-line reps there are sometimes not very helpful. If so, be persistent, and keep at them until you get a satisfactory answer.