With a decent ISP, it’s not that hard. All you need basically is the ability to have a public IP address and have a server behind it. Static IP definitely makes things easier, and having reverse DNS on your IP address plus a bunch of DNS configs (PTR records etc) will help to make sure your email is not marked as spam.

For software, I’m using Kolab. Check it out ;-) But yes, the ISP is the hardest part.

(Not to mention that Google has insance policies regarding IPv6)