Looking for some advice and guidance please? Which free Instant Messaging server & Email server is best for SUSE 11.4?

I have an office with a mix of approx 20+ Windows 7, Macs desktops, and we want to bring our email and instant messaging services/solution in house.

Im technical, so am not afraid to dive in feet first, but being relatively new to Linux can anyone advise a good free product(s) which would work in this kind of setup bearing in mind the windows and mac desktops need compatible clients?

Any help and guidance is appreciated.