All of a sudden, one of the regular users cannot receive external mail. There is no change of address, I have checked the e-mail addresses. External message, when tracked get past the firewall (Cisco), but a NDR is sent. I can't figure it out. Any help would be appreciated.

Hi, It would help if you posted the NDR (you can change the names if you want) Leif

Ask user to send a test mail from your org to his/her personal external smtp(hot mail or yahoo) ,and reply to that mail itself,instead of creating a new mail,then see what error user gets on external mail id.