97% of emails reaching IronPort/Domino are Spam

As silverfire indicates, it depends on two things:

• How many emails are we talking in terms of numbers (10,000 or 100 a day?)
• How much bandwidth you have connected to your mail server

The only thing you can do to reduce your bandwidth is to use an external spam filter, such as Postini (which is a Google service, who we have had success with for some of our high-volume spam domains). The question is really is it worthwhile, and if you have a successful anti-spam solution now, then it all depends on the answers to those two questions above.

Also consider you can host SMTP servers in one or more datacentres then relay back to your servers in house. As per Mark's answer, Postini would be more cost effective form of this arrangement.