[exim] no ehlo possible when sender IP has no hostname

hello

i´ve a problem with auth configuration on exim4
i want the server to relay mails from authenticated users.
the option host_lookup = *  , so the server does host lookups on every host

when the IP address of the user has a ptr in DNS and can be resolved, ehlo is
possible, and the authentication works fine

when the IP address of the user has no ptr and the hostname of the IP can´t be
resolved, the server responds to ehlo command with status code 502, only helo
is possible, so no auth advertise and no authentication is possible

i can´t find the option that makes this happen

i´m new to exim and i would be very grateful if anybody give me an idea where
to look after the problem


thanks in advance
roman