It's been awhile since I looked into it, but I recommend outsourcing your email.
Companies like fusemail (http://www.fusemail.com/solutions/resellers.html ) will give you accounts at $0.69/month/account for a 1GB account (last time I checked anyway). They provide an API and a dashboard for managing it. They also handle backups, I believe (read the fine print before you trust me). I think there is spam filtering and anti-virus built in too.
Kept thinking that as well. Only issue is that 0.69 x 2000 equals a pretty good chunk of change for us. One thing I would reccommend though is putting it in a colocation facillity rather then local. Also start with a beefy machine because its a real pain to upgrade later.
Matt