[Exim] format of domain names

exim has a lot of stuff like

    helo_strict_syntax option

    Option: helo_strict_syntax
    Type: boolean
    Default: false 

    Because so many systems have been found to use underscores in the names
    they send in the SMTP HELO command, Exim by default permits them, though
    it is not in fact legal to use underscores in domain names.

when, in fact, rfc2181, makes quite clear that there is no restriction on
the characters used in domain names.

randy