I find the worst part of running a private mail system is the spam. It morphs so often that you are always fighting a running battle just to keep it under control. Achieving a zero-spam target is almost impossible. The larger players (gmail/Office365) only manage to do it because they are handling millions of messages a day and have thousands of honeypot addresses just for spam catching. As a small player there's no way you can compete with that.

My anti-spam system is a relatively new install of SpamAssassin. All my incoming mail runs through that. It's about 80% effective. I've not had the time to teach it properly or tweak the bayes filtering so I'm sure it's not running as effectively as it could be.

I'd like to rent just an anti-spam gateway service that my MX records would point to and it does all the filtering and then sends good mail onto my private mail server, but the costs I've seen so far make it uneconomical for me.

If I was starting over, I'd pay for Office365 small business tier, which gives me cloud Exchange plus mobile sync support for mail/calendar/contacts/tasks. Most other providers can't supply an integrated service that works with Outlook.