I would assume for their millions of paying email users... The business model would be to charge money for feature requested by customer.

If you think that enterprise hosted email customers want end-to-end encryption, you are seriously mislead. What most of the customers want more than anything is censorship and compliance, at delivery time. They want mails to be rejected if they have naughty words. They want viruses dropped. They want messages containing customer credit card numbers dropped.

Content analysis is the #1 feature required by enterprise email customers. They do _not_ want end-to-end encryption that would prevent content inspection by intermediaries. They do not want that at all.

I guess it all depends on the enterprise. Encryption is a requirement for certain businesses. My partner is required to use it at her law firm when working with certain clients.