The only "hard" part I find with running my own mail server for the last 25 years is switching platforms.
Deriving a large part of my income by supporting Microsoft software in small business environments, I have been running Exchange versions all that time. I have enjoyed Outlook as a primary business tool, and looking at the conversion strategies to move over to a *nix mail server that will support that client and the 25 years of horded emails I now store (don't mess with my jokes folder, dude) has been the most daunting part of it.
Even though I have been on Debian releases as my primary workstation OS for over 12 years now, I still have virtual Windows sessions to keep using Outlook. And granted, that also lets me simulate what my clients "see" when I am assisting.