[exim] acl-smtp-helo

hi,
i tried to define acl-smtp-helo (found one from an old vexim installation):

acl_smtp_helo = ach
begin acl
ach:
accept hosts = :
deny message = "helo deny host='$host'"

the log tells me host='' (empty string) when messages come from fetchmail
exactly this i expected and i thought that : is the list containing
exact one empty element ?!
or is variable host simply not set in this situation ?

? address = user @ [ host .] domain
in what situation is $host emtpy and why ?

thanks for help
andreas

please, have a look at my first question, too.