Ask HN: Alternative to Exchange for Email Server System?

Hi, I'm looking for recommendations for an email server system. I work with an institution that does alot of communication through email, and as such it's a crucial part of their tech infrastructure.

The requirements are the following:

- Availability is essential

- It doesn't have to be open source, as long as it's not outrageously expensive. Also, it can be windows or linux based.

- An API access for email retrieval would be good, but if there aren't, what's the best choice between POP and IMAP?

So far, we've glared into Zimbra that seems to be very recommended and looked into some other solutions such as mailenable and icewarp.

Thanks in advance.