Well for starters, ditch Sendmail, use Exim, and then implement DRBD with linux HA, Dovecot, and Perdition.
If by enterprise grade, you mean "good enough for an ISP to use", then open source does just fine with a bit of tweaking and fine tuning.
So spamassassin uses a lot of CPU? Have you seen the resources an exchange DAG requires?
Granted, exchange has a nice calendar system and the extra goodies that businesses want, but that's a real cost/benefit analysis given licensing and the amount of hardware you have to throw at it.