I'm doing some analysis on Exchange Online Encryption vs Zix. Our organization uses Zix and and we are also on Office 365 E5. With the advanced security built into E5 is there still a need for Zix Encryption, or is Zix superior in its encryption algorithms and features?
I don't have any experience with Zix so I can't make any comparisons. However, I can give you my thoughts on Exchange Online message encryption. It's pretty cool that you can send an encrypted e-mail just by using a keyword in the subject. At least, that's how our transport rule is configured. On the other hand, while the sent e-mail is encrypted, the copy that's stored in your Sent Items is not. Anyone gaining access to your Sent Items can clearly see the "encrypted" e-mail's contents.
The workaround for that is to send yourself the encrypted e-mail, open that, and then forward it to the intended recipient. Not pretty but it works.
I think, that if you're happy with Zix, you'd be better off staying with them....at least until the Exchange Online version of encryption matures.