Some E-mail not getting through

We are currently using MS-Outlook. Message our customers are getting is;

"Doesn’t recognize address".

Some items are marked;

: [#5369] FW: Warning: message 1XXcRI-00029m-2y delayed 72 hours

We transfered our email domain to comcast some months ago but we keep hearing from clients that they sent someting we did not receive. Can you tell us what is going on? We are using a vertical application from which we can send email. The Bcc copies are not working either. Please advise