The Outlook Connector. It is not ready for prime-time. Oh it works, but is clunky if you want to utilize Microsoft Outlook as your email client. To be fair, Google Apps Sync was just as bad. I suppose any Outlook connector can never do what is built into the Exchange and Outlook ecosystem.
Also, initial deployment requires lots of attention to detail, whereas a cloud service is more set-it-and-forget-it. As the sole administrator I find myself spending lots of time administering the system until the first month or two are history. Or until users just get used to the new normal.
If you plan a IMAP-only or webmail-only email system, this is an amazing solution considering the low cost and excellent support. The hardware demands are minimal, from what I've observed on my virtual server Win2016 on ESXi on a NAS (SSD/HDD 12-drive array).
I decided to pay the $500 annual priority support after purchase because I found I was going to need some handholding during the honeymoon phase. Totally worth it. They are Americans, speak English, and I get to them in less than 5 minutes where they're solving my problem.
If you've been sheltered by managed email services, prepare by running MDaemon in trial mode and implement it on a different domain to see how the email works from start to finish. That was really helpful in comparing systems and experiencing the ease of administration.
If you want calendar integration like Outlook, you'll be using the Connector which is frustrating for our users who are used to Outlook/Exchange integrated smoothness. To be fair, MDaemon is continually improving and implementing suggestions so I'm sure the connector will be greatly improved over time.
Spoofing was always a problem with Office365 and Google Apps. MDaemon has integrated modules that handle this. MDaemon has built-in security tools that help protect users and the server itself. Cost was another issue we overcame. Monthly licenses cost less than $0.30 per user compared with $8.00 per user with cloud-based email services. And we finally broke free of another subscription-based service, meaning we can keep using all email services even if we don't pay for maintenance fee next year!