E-mail solution for 3k users?

Not heavy users, perhaps 300 of such are heavy 9am - 5pm.

Microsoft exchange out of the question, can't get approved for it. :(

Kinda just surfing around right now, google so to speak.

Figured I would post this as well.

Have tried ATMail and Sogo - both cheap solutions that are linux based... not really going well during the testing phase... Open to Windows or Linux based solutions if you know of any..


Need more information: Need calendaring? What clients to connect? Backup/retention requirements? How many offices?