"most business users use Exchange"
Well, if you've paid for an enterprise-wide licence for everything Microsoft do, then probably. However, I'd be surprised if most SMEs weren't using just Outlook or Thunderbird with an email server running on a Linux box. There are plenty of howtos for setting that up and for some companies (or, equally, their customers) there may be a legal requirement to avoid routing all your private correspondence through the US.
Biting the hand that feeds IT
