[Exim] Syntactically invalid hostname

Hello,

I have those messages from time to time :

2002-08-22 09:08:05 rejected EHLO from [62.115.227.131]: syntactically
invalid argument(s): gbc_ex001.gainsbc.co.uk

The incoming message gets rejected.

I guess that's because of the underscore. Unfortunately, I cannot force
people to change their naming. Is there an option in Exim 4.04 to ignore
those errors and accept the message ?

Thanks,

Yann.