Why not? A bridge between SMTP and Nostr should be possible. Imagine being able to upload an old .PST file to a relay and having all of the emails and events contained suddenly visible.
Discussion
It's absolutely possible.
Don't make it free though. Both Workspace and Proton are paid now for custom domains
.pst is Outlook backup, right? Or does it work for all email clients?
Yes, .pst is out of Outlook, and I meant it as an example, not a catch all. MSG, PST, EDB, OST, EML, MBOX, and probably many other different email file storage formats could all be integrated at some point. And I would love to support the devs that make that possible.
Proton allows you to import mail from Gmail, so I suppose the solutions already exist