It would be useful if we could use ProtonMail as a SMTP client, so from an account like account@proton.me, I could connect to the Google SMTP servers and send email as account@gmail.com. This would allow full support of both email addresses indefinitely while maily using the ProtonMail applications (I would never open GMail), but also it would allow to test ProtonMail for a while, with no risk, while considering migration (GMail would still have all the emails in there, since it stores emails sent through SMTP).

This is similar to how GMail can use third party SMTP to send email with alternative email addresses.