Closed source or not, the trust comes from provider serving you the same payload. Even if it was open source it could be changed. So if you care about that you

Closed source or not, the trust comes from provider serving you the same payload. Even if it was open source it could be changed. So if you care about that you can't trust any webmail. Instead use Thunderbird/mutt/whatever and GPG/PGP, like you could for the last 20+ years.