We're looking more for reliability than high volume, but you never know what the future holds. The thing is, we're an IT support company and we already do web-hosting but a number of our customers really rely on e-mail and would like to use our services.
We just need to be sure that our reliability is top-notch. Rackspace appear to use something like Icewarp, which supports failover - so that may be our way forward.
Yeah and the issue with Exchange is the costs really outweigh the feature-set. You would need Exchange CALS and then additional blackberry CALS for Blackberry users.
A couple years ago Exchange was the only way to connect Blackberries up but now days, Exchange doesn't really provide a step-up over other offerings. My recommendation would be Icewarp since it works well with Blackberries and Windows mobile phones. Pricing is really close on either and both are cheaper than Exchange.