o SMTP = postfix/exim/sendmail (hint don't use the last two)
o dkim, spf & reverse-dns are just DNS records. Spf is a list of IPs that your domain "owns", dkim is a hash that proves you own the originating server.
o Log parsing is standard log interrogation stuff. You are better off writing a custom metrics exporter that talks to the queue than parse logs routinely.
o Greylisting is a configuration option for email relays where they reject the first attempt of unknown servers attempting to deliver mail. nothing more to it. (https://www.greylisting.org/implementations/postfix.php)
o rate-limiting is again a config option: https://www.vooservers.com/technical-blog/rate-limit-outboun...
o spam filtering: https://docs.iredmail.org/enable.dnsbl.html is a good start
There must be an inflection point where it becomes more economical to host your own, but I doubt 22k an hour is that point? Maybe it is.
SMTP, relays, MX-records, SPF verification, DKIM signatures, spam filtering, reverse-DNS, blacklisting, "Internet Message Format", throttling, MIME-encoding, email-antiviruses, bounces, headers, log-parsing, rate-limiting, greylisting, RFC standards (and that no one follows them)... And of course - about IP address reputation (more on that later).
Are there any open source tools to tackle some of these? Given the maturity of email marketing I would expect some pretty robust free tooling available.