[Exim] Allow none standard characters

Hi All, I have been digging around with no luck on how to allow more
then one none standard character. I already have one entry
"helo_allow_chars = _" (no quotes). This works fine however now all the
mobile phones in Japan have email and the users have been advised to use
unusual email addresses to avoid SPAM. Now we are getting a lot of
complaints from our user about not being able to send email to the mobil
phones using "/" (no quotes) in the address. I tried "helo_allow_chars =
_ : /" (no quotes) but this didn't work. Can someone shed some light on
this for me, please feel free to email me directly. Thanking one and all
in advance.

Best Regards
Fred Krug
Hunter Point Online
tech@???
Tokyo, Japan