[exim] reject-log question

Hi

I thought, I understood most of exims logs, but now I found a entry:

2004-09-15 21:34:58 rejected HELO from [62XXX41]: syntactically invalid
argument(s): nt_49613199.intern.XXX.de

The XXX parts are replacements from me to hide some clients information.

Can someone explain me those line?
What is syntactically invalid on this?

Regards,
Steffen