My employer has never paid the Microsoft tax.  Our email systems are based open standards technologies (IMAP and SMTP).  We have a handful of email servers that support over 40,000 users with zero downtime.

Daring Fireball: Exchange ExchangeThe pressure isn’t going to be on Apple to license proprietary technology from RIM or Microsoft. The pressure is going to be on IT departments to support open standards like IMAP, which the iPhone supports.