I self hosted for 20 years, worked flawlessly, gave up because of security concerns. I would like to go back to it.

Question: How do you manage the security on such a box? Is there any simplification I missed?

I couldn’t keep up with it. So many patches, unrelated to mail, broke something in the stack, bringing the server into a critical state. Often, I had to lock down everything before going up again, consuming a day’s effort or two. These were two days without mail.