Client Cannot Email Me - Getting kickback saying PPE Hosted tried to deliver your email message, but was unable to do so

Hello,

One of my clients that has been emailing me for years has not been able to the last 4 days. 

This is the error message he is getting

This is the PPE Hosted Dispatch mail system.

PPE Hosted tried to deliver your email message,
but was unable to do so for reasons outside our control.

We recommend contacting the person or service responsible for
looking after the recipient domain's email server directly.

The last error message we have available from the recipient's
server is included right below, along with an extract of your
email:


<xxxxxx@comcast.net>: host mx1.mxge.comcast.net[] said: 552 5.2.0
    zmDhtRsZOVAPLzmDhtcylZ Prohibited by policy (in reply to end of DATA
    command)

Any Idea how to get this fixed?