I need this feature too. I am currently using a custom domain. I have a mail forwarder on that domain to redirect my mail here. When I click "Reply" to any of those messages, it uses my protonmail.com address in the "From" field even though the email was sent to my custom domain address. I can change it manually using the drop-down box, but that is a PITA. There needs to be an option to use the original "To" address as the "From" address when replying. FYI, I cannot change the MX record of my custom domain to point here because there are other mailboxes that must remain there.
I need this feature too. I am currently using a custom domain. I have a mail forwarder on that domain to redirect my mail here. When I click "Reply" to any of those messages, it uses my protonmail.com address in the "From" field even though the email was sent to my custom domain address. I can change it manually using the drop-down box, but that is a PITA. There needs to be an option to use the original "To" address as the "From" address when replying. FYI, I cannot change the MX record of my custom domain to point here because there are other mailboxes that must remain there.