The Zimbra Open Source Edition is probably a very good choice.
- 99% of all companies don't need features then the open source edition.
- it supports large mailboxes very well. ( some of our employees have 21gb mailboxes, it still runs smooth )
- You can buy a plugin for encryption if you really need it.
- Mail (IMAP), Calender (iCal) and adressbook (LDAP) sync is possible to almost any device.
- You can always get the commercial version if you need the extras.
I don't think you can remotely wipe your mail using an open source product but nowadays you might simply get any android of iphone device and use a wiping app. Maybe not as convenient but it works.
Spamassasin can work very well ( it certainly does for us ) using external blocklists and distributed mail analysis services ( dcc, razor2 ) in addition to it's core filters. We added greylisting as well. Everything runs as part of the Amavis product. We don't use Bayesian filtering though. While good on paper we found it to be to unpredictable in real life. ( people reporting valid mailing lists as spam instead of unsubscribing, etc ) Instead we added around 15 additional custom spam filter lines over the years but that's it. Now all our spam is gone. We filter mail for over 1500 domains and our customers have never been happier.