Amount of hours spent to keep a default #chatmail onboarding relay with 500k active addresses running: near zero.
Glancing at stats sometimes: two million messages per day. CPU and IO load at 20% max. 500gb raid1 ssd meandering around 60 percent full. There is room for various optimizations but no big need currently.
end-to-end encryption with metadata minimization is best if servers are boring. No clustering or enterprise license needed, no "SRE team" either.