We needed a stamp.

Regulated "Emails cost 1 penny" would have worked fine. All you need to do to meaningfully fight spam is have a cost that isn't completely negligible; Spammers started out at a rate where they spend less than a day's wages to message literally every human being on the planet; At those costs even finding a single person you can convince of your Nigerian prince account nets you a profit.

We controlled the pipes and the formats in the 90's and 00's almost unilaterally. We should have made a stamp.

That did work for decades up until recently. It took me a bit to realize that google email recipients had stopped receiving my emails.

YMMV but I never had issues with Gmail accepting mail from my personal server. And I didn't even do anything Gmail-specific, just standard SPF+DKIM and making sure my server is not an open relay etc.

Microsoft on the other hand...

I self-host an email server and can definitely send email to Gmail addresses.

I've self-hosted email systems for businesses for nearly 20 years. I've actually had far easier times delivering to Gmail/Workspace clients than Outlook. Outlook constantly breaks strict DKIM with some of their protection scanning nonsense for emails that seem to get good deliverability almost everywhere else.

Been doing it for over 20 years without issue, for myself and many other customers.