[Exim] What does this error mean?

We're running Exim 4.0

We have a few client who are unable to email us and from one we get this
error message in our mainlog for exim

2002-09-27 16:14:42 rejected HELO from ral-exch.fminet.com [208.17.36.8]:
syntactically invalid argument(s): ral_exch.fminet.com

Can anybody tell me what a "syntactically invalid argument" is?

Thanks
Frank