Typically I'm a DIY type who loves tinkering and building...

HOWEVER, I have learned the hard way to never apply that spirit to email.

In Europe you see this stuff all the time with old school "IT" (what old industrial companies call tech) people balking at the prices of commercial API-based senders and email marketing ESPs.

"Money to send emails in the cloud? HAH! Back at Siemens in 90s we were running millions of emails out of our servers just fine!"

Nobody understands that deliverability has gotten immensely harder these days, and trying to DIY it if its not your core business is just plain stupid. I would never in a million years try to roll my own email, it's nightmarish legacy cruft and footguns all the way down, in everything from IP/Domain Rep to something as simple as the HTML in the email templates themselves.

Microsoft Outlook and Gmail have the last word on everything in email, and their defacto duopoly (over B2B and consumer email respectively) means you play by the rules they set in 2008 and are too lazy to change or you don't get delivered. The protocol of email exists separately from the world of the actual inbox providers, which are locked down to insane degree given the security/spam concerns with email.

Google is at least less arbitrary than Microsoft. Microsoft will decide an email is spam today, and tomorrow the exact same email is perfectly fine. I think Google relies more and more on sending IP and domain reputation rather than content.