My understanding from the article is that correspondents are not encrypting the mail, but that Protonmail is. So unencrypted messages from Fastmail are being en

My understanding from the article is that correspondents are not encrypting the mail, but that Protonmail is. So unencrypted messages from Fastmail are being encrypted to a key that Protonmail found on a random keyserver when that email hits the Protonmail server. This is a common feature provided by privacy oriented email providers. The idea is that the provider will only have access to the unencrypted email at entry to the server but not after that. The email of course can't be signed in any meaningful way, but the original unencrypted message wasn't signed either. This strikes me as the sort of thing that you would want the user to specify a particular key for, but I suppose Protonmail is attempting to increase usability by making it as automatic as possible.