As of July, ProtonMail is (finally) fully interoperable with other PGP mail clients.
You can export your ProtonMail public key to non-ProtonMail contacts, so they can send you encrypted mail using their client of choice. You can import their public PGP key, so you can send them encrypted mail.
Once that's done, email encryption can be automatic between the two of you with only the one party using ProtonMail.
They have a public key server, so an external user's client can automatically look up the key of any ProtonMail address.
https://protonmail.com/blog/address-verification-pgp-support
https://protonmail.com/support/knowledge-base/how-to-use-pgp