If you own multiple Proton Mail or custom domain addresses, you can select your default email address.
Your default email address is the address that appears by default in the From field of the message composer, although you can always select another address to send from.
Your default email address is also the first email address you see when you log in to your Proton Account.
To update your default email address in the Proton Mail web app:
1. Log in to your Proton Account at account.proton.me(new window) and select Settings → Go to Settings at the top right.
2. In the left sidebar, select Identity and addresses. Scroll down to My addresses.
3. In the Actions column, click Make default next to the address you want to make your default sending address. You can also drag and drop the address to the top of the list to make it your default sending address.