Temporary Envelope Failure

My friend said he got the following error when trying to email me:

Unexpected recipient failure – 421 temporary envelope failure (#4.3.0)

Is this a problem with his email sending machine, or a problem with my email server?

I have just had greylisting enabled on my server and wondered whether this is causing the problem. I can send and receive fine, so I hope this is a simple setting on his side, and any assistance in determining what it should be would be most welcome.