[ipodopt]

I have used both pm and migadu with custom domains. Miguda ends being much better on that front feature, pricing, and ease of use wise.

https://www.migadu.com/pricing/

You do lose the layer of encryption protonmail provides. But it is essentially boils down Encryption at Rest and e2e when between two protonmail users. It can be mimicked by pulling down the mail from the servers to your machine and using pgp.