
Hello,
I know this may sound like a pretty simplistic answer, but has your user checked to make sure he/she has the recipient’s address correctly listed in his/her contacts? I’ve seen many times where a user will set up a contact and fat finger the address or simply exclude a character making the address invalid thus returning your stated error.
Good luck!

Are you using the exact same address for this person as the user is using? I.E. are you replying to the same message as theuser would, or are you sending a message based on the address the user has given you?
The problem could be, if the latter, that the user is giving you an address in the form of recipient@domain.tld, while the return on th ee-mail is recipient@servername.domain.tld. If you do an MX lookup on th eformer, you’ll find an MX record, on the latter, there will be no MX record.
If you are both using the exact same address, then, we would do a bit better with the exact SMTP error number that is given in the error message to determine what may be going on. It is possible that the user may be blacklisted in an anti-spam filter at the user’s site, which would be why you can send the message but the user cannot.
Steve//














