Pros for Zimbra are:
It supports Exchange Web Services and ActiveSync so you get complete feature parity for setting up Macs/iOS and other Exchange supporting devices. Makes it as easy setting up users as a full O365 setup.
Licenses are pretty cheap, we're paying I think £400-500 for 25 users per annum.
The web interface is good and has very strong feature support for aliases, mail filtering (O365 filtering is abysmal).
Supports adding External mail IMAP/POP accounts so you can send/receive all your Gmail emails from Zimbra.
It is very standards compliant with excellent IMAP, CalDAV, SMTPS, POP3, EWS, ActiveSync support. No half baked support unlike IMAP on Exchange or Gmail which occasionally does something protocol hostile.
Cons:
Zimbra web interface is not as good as Gmail or O365. It's okay though and feature rich.
If maintaining your own deployment expect some hours put in setting up the usual stuff like SPF, DKIM, DMARC, LetsEncrypt.
Standard support is slow although they do eventually get the issue fixed.
I don't believe Zimbra supports anything like O365's Shared Mailbox feature.
Spam filtering is not great, certainly not compared to Gmail.
