[Mailman-Users] Member cannot post

[PeteBell]

I have a member whose emails to my list address are being returned to  
him with the following message...

> Remote host said: 550 Administrative prohibition

I have NOT blocked his address and there doesn't seem to be anything  
wrong, or wrongly setup, with his subscription.

What possible reasons could there be for this, where do I look for  
any clues (I do NOT have access to the installation), and how is it  
rectified?

Cheers
Pete Bell, UK