NKCbooks

Not sure if this has been posted before, but just happened today: after
replying to an email from a contact in my address book, my reply bounces
back with this message: permanent failure: other address status
I have sent emails to this contact before without problem. Any suggestions
or ideas what is up? I looked at the contact in my address book and he is
only listed one time/one name, etc.

Thanks,
NCbooks