It was Exim on Debian; generally it would chug along fine for months and then incur some sort of regular sysadmin issue that nonetheless needs dealing with. Upgrades. Occasionally, breaking upgrades. Changes to anti-spam rules on other systems causing deliverability issues. ISP mandating IP migration. Earlier in the process it was an actual, physical rackmount server not a VPS, which incurred extra effort a couple of times to replace it.

And the big one: inbound anti-spam.

It's not that issues are frequent, it's just that there's no guarantee they won't be badly timed. There may come a point in your life where (a) you don't have a lot of free time and (b) you badly need a specific email.

I still have the mail domain, I just pay an ISP to handle it for me.

I heard a lot of people saying hosting your own email server was the best way to be considered as spam by every big email provider. I think it's a big issue, but I've never actually tried it.