Horde Imp should have at least 'enscript' and 'pgp'/'crypt' use flags, so that it install the sofware it 'needs'. These were the ones that I found so far, been using it for a week.
and what software should it be pulling in with those USE flags ?
Well with enscript, enscript sould be emerged, with crypt/pgp pgp should be emerged, what other info do you need? Look at the Squirrelmail package, it does part of this.
ive added crypt? ( gnupg )