[soromak]

Originally Posted by sflorack

I would assume that any provider/client that uses IMAP IDLE would fit the bill. Wikipedia has a list of clients on their site: https://en.wikipedia.org/wiki/IMAP_IDLE

IMAP IDLE would not work with iOS devices.
Any provider with ActiveSync support would fit.
If you are after a more affordable provider, EuMX works quite well with Horde ActiveSync.