That would be really awesoem and give an edge on all of the other webmail services. But I think all in all there are a few quirky things that need to be functional first. Or simple featrues liek a distribution list ability(or grouping of email address as a distribution list.) Maybe even the ability to do blacklists or have a spam filter ability. I thinkt he basics would be best for now. I can not lie I really do enjoy the newest SVN of RoundCube. We use it as our ISPs webmail. It works great jsut a coupel little quirks as I have mentioned before.
-Cheers!
DesertAdmin