When users sign up for services (e.g. Facebook) they might use a random generated email alias like facebook.com.spar731@passinbox.com, note that the domain of the website who this alias was intended for (facebook.com), is in the user portion of the alias and can be used as a form of data origin authentication like so:

Whenever a user receives an email, the proton mail client checks whether the domain of the sender is present in either the user portion of the beneficiary of that mail (FACEBOOK.COM.spar731@passinbox.com) OR that the domain of the sender is in some sort of whitelist coupled to the domain in the user part of the alias.

Regarding the latter, let's say the user (or community or AI?) whitelists meta.com as an alternative for facebook.com. A sender with email helpdesk@meta.com would also not be considered spam when the beneficiary has facebook.com in the user portion of its email alias because meta.com is in the whitelist for facebook.com.

Another feature that could be built using this mechanism is creating inbox folders coupled to aliases. For example, you might want to generate and re-use your alias socialmedia.app834@passinbox.com across facebook, instagram, snapchat etc. And create a dedicated inbox folder to group all social media-related mails together and such that your main inbox does not get tainted with social media-related ads.

I think a simple check that compares the sender's domain against the user portion of the beneficiary's email and showing a small check mark based upon that is already a small feature but would be really helpfull