Initial setup took a few days of experimenting, that was back in the days of Sendmail. After that is ran fine for a while until spam starting becoming a problem

Initial setup took a few days of experimenting, that was back in the days of Sendmail. After that is ran fine for a while until spam starting becoming a problem. I eventually implemented a spam filter with Spamassasim with a nightly cron job to train it with spam which made it past the filter. After a few years I added greylisting to the mix which ended up complicating matters a bit due to some common mail hosts (Google and Microsoft being the biggest culprits) breaking the concept of greylisting by using different mail hosts for resend attempts. I made a script to add all SPF-allowed IP ranges for a domain to the whitelist which solved that problem. Total time spent on the mail infrastructure over these 22 years is negligible, probably ~8 hours per year.