They have an opt-in feature that encrypts all emails that will be saved in your inbox with an PGP key pair, like Proton does. https://kb.mailbox.org/en/private/security-privacy-article/activate-your-mailbox-org-guard

No mention of MailboxORG?

You have tabs on top of the download button to change the platform.

I think you got the wrong link, the downloads to Mullvad Browser are here and it’s not on Alpha.