For those who use the EXIM mta (mail-mta/exim) there is a very handy side package called exim4u. It enables virtual email domains, spam scanning, virus scanning, and more. It's VERY handy. I'd really like to see a copy in the gentoo repository. The package can be found at www.exim4u.org. There is some documentation there, although it's very old and pointed towards (I think) debian. Still, it would be wonderful to have in the package library.
The long list of requirements is here: https://github.com/Exim4U/src/tree/master/docs