Question : Undeliverable email...strange error message

A person at our company is getting the following error message when sending an email to her boyfriend.  She has been sending him email at this address for the past 3 years and has never had a problem.  Nobody else is receiving this message as far as I know.  


The following message to was undeliverable.
The reason for the problem:
5.1.0 - Unknown address error 551-'5.7.1 Receive of data is denied'

Any ideas what this is and how I can troubleshoot it?

Answer : Undeliverable email...strange error message

It means that it was rejected by the protocol filter. Do you or the other side ([email protected]) have a protocol filter on, maybe a spam filter, or something like that