I have now been using OS X Servers + iRedMail on an old mac mini I had kicking around to handle my e-mail services. So far so good.
Some key points:
- Unlike iCloud, unlimited server side email filters can be setup within iRedMail and these rules can be far more complex than iCloud allows.
- Push e-mail works on the inbox and inbox only.
- Contact, Calendar, and notes all sync without issue.
So far no issues whatsoever.