There is significant lock-in with email because:
1 - Email migration sucks and if you're a big org, this process can/will be complicated, expensive and carry potential down time for end-users.
2 - The bigger you are, the less willing you are to go through with #1 without a really, really ridiculously good reason
Migration is so painful some companies are still emailing from unpatched Exchange 2007 servers under the IT guy's desk.