Seems pretty self explanatory.
Before the body of the message & attachments (if any), there has to be a valid &to:& (recipient)..... can you find out what precise address they are sending to?
They may be sending to an address with bogus characters, or something that is not understood by the exim (smtp) server on your CPanel system.
You also might try tailing the exim_mainlog (/var/log/exim_mainlog), during their connection to see what else is reported there.
Before the body of the message & attachments (if any), there has to be a valid &to:& (recipient)..... can you find out what precise address they are sending to?
They may be sending to an address with bogus characters, or something that is not understood by the exim (smtp) server on your CPanel system.
You also might try tailing the exim_mainlog (/var/log/exim_mainlog), during their connection to see what else is reported there.