[Mailman-Users] message body is blank for some users

[Heal Secretary]

Hello

One of my users' messages are always sent out to list members with a 
blank body.

I have tried unsubscribing him and resubscribing him. Same result.

Most other members are posting fine (though some never bother!).

What could be causing this and how can I run diagnostics to trace the 
problem? Or, even better, how can I fix it?

I am running Mailman v 2.1.7. I am not allowed to upgrade to 2.1.8 as 
the installation is provided by a third party (CPanel) via my web host.

Any ideas?

Many thanks
Nigel