Proton is currently the only one among the popular web‑mail services that claims to offer OpenPGP inter‑operability. Indeed, for a new user it is sufficient to open a "free‑tier" service account, and send a correspondent their e‑mail address. Any existing OpenPGP user - specifically someone not a Proton service subscriber, but somene using an Open-PGP capable mail‑client program (for instance, Thunderbird) can expect their mail program to automatically fetch the public key required for encryption of messages to be sent to any Proton mail address. This user can attach their public key to the message to Proton user, who is then offered the opportunity to accept the attached key for encryption of subsequent messages sent to the external user.
However, for a Proton web‑mail user to initiate encrypted communication with an e‑mail address that accepts only OpenPGP encrypted traffic, it is necessary to manually add the "external user's" public key - even though that key is available via WKD protocol or from keys.openpgp.org server. This makes Proton's claim of OpenPGP inter‑operability only partially true.
To add the external user's public key to your Proton account, you will need to perform some additional steps:
Assuming the external user has uploaded his or her public key to the keys.openpgp.org, go to: https://keys.openpgp.org/ and enter the email‑address into the search bar. If the key is known to the server, the browser will offer to download a file with a name consisting of a long "key-fingerprint" string and .asc extension. Save the file locally on your computer. (The process will be similar if the public key is posted on a web‑page where it can be downloaded from, using your browser).
Once the correspondent's public key is stored locally on your computer, log into your Proton account. (The following assumes browser GUI interface of Proton mail).
In Proton Mail:
And that's it - you can now send an OpenPGP signed and encrypted message to your contact - even when he or she is not a Proton mail user!