Currently, when you open a Contact card and click "Compose", it opens a new e-mail in which the "From:" field is set to the same default e-mail address of your Proton account.

Proposing the addition for a "Correspond From Address" field for individual contacts in which, upon clicking "Compose" on that contact, the From: field will be set to what you had set to that contact card's "Correspond From Address" field.

Right now, you can only sort of fake this by adding a Note in the contact to remind yourself "Contact this person using X address", but you still have to pay attention to that and then manually set the From: field to what the note said. What I propose should prevent forgetful "oopsies".