As a company looking to self-host, however, your product looks very appealing, but then your battling other players than just GMail. I think in the corporate sector, it would be very hard to dislodge Outlook, which is a shame really.
Despite Google's privacy policy and ad behaviour (yadda yadda), they ARE still a trustworthy company, regarding uptime , backups and everything technical. Though I remember them losing quite a few accounts about a year back. But still, your point is very valid. Why would anyone trust a small company (or even single hacker) to maintain your precious emails.
* A very straightforward exit strategy: make it easy for users to at least extract their mail from your service, if they host it on your servers, and want to have an offline backup.
* Offer an option to license for self-hosting: hosting their own email could take away their fear of uncontrollable downtime.
I'm not sure how hard it would be to take some Outlook Web Access share away if targeting smaller companies since the cost could be potentially lower and licensing agreements potentially much easier to understand.