[exim] / in userpart

Hello,

a customer of us asks us to allow a slash in the userpart of a
forwarder-target. The default configuration of exim excludes this
character and even while I know how to allow it, I'm not really sure if
I should actually allow it. Is this RFC conform? Is this good practise?
We also allow # and + sign which doesn't look correct in my eyes but
does work technically.

Regards
Marten