I was hopeful that https://www.nylas.com/ https://www.nylas.com/ would be the de-facto "adapter" placing a common API surface on top of the major providers and

I was hopeful that https://www.nylas.com/ https://www.nylas.com/ would be the de-facto "adapter" placing a common API surface on top of the major providers and dragging them into a modern-API world. They even had an email client of their own as a proof of concept (forked by one of the original authors as https://github.com/Foundry376/Mailspring https://github.com/Foundry376/Mailspring - and its reusable core https://github.com/Foundry376/Mailspring-Sync https://github.com/Foundry376/Mailspring-Sync may be interesting to many here). But they've pivoted towards making their API only available behind B2B contracts and opaque pricing, and primarily used for corporate email monitoring and CRM use cases - perhaps because security and privacy considerations are nontrivial. I'm still rooting for them but it's a shadow of what it could have been.