[exim] server with underscored Helo

Some user had voicely complained me that one of thetir correspondent
(say mario@??? -fictitious-) has all his e-mail rejected.
I checked and turned that the server, that officially has the name
mail.ditchedserver.org, send this helo:
HELO ISCRA_domo.arsi
(and this name is the real one that send !)

my exim 4.63 is unhappy and reject immediately the connection saying there
is an illegal (I suppose '_' )characther in helo name.

This sound reasonably since all but that server that send with such headr
send malware. However these users are veri upset and do not accept that
our servve cannot accept such messages, while other servers in other
istitutions accept without problem.

Then i ask: is possible to skip, for a certain source IP, and for a
certain set of recipient EVERY check, acceptin even the most malformed
message ?

In alternative: can some one give me a link to some offical rule saying is
not acceptable sending Helo with underscores or non existant FQDN ?


--
Leonardo Boselli