[exim] syntactically invalid argument(s)

Greetings!
Whenever my server receive an smtp transaction from a host named in the following way "name_host", it rejcts it with the following message.
rejected EHLO from host76.200-71-231.telecom.net.ar [200.71.231.76]: syntactically invalid argument(s): server_notes3.alumni.iae.edu.ar

How do I solve this situation

Eduardo