This would leave Bridgy free from being the potential source for security leaks and put the onus on the end user. You’d naturally need to have the ability to reset/change the user’s hash in the case that they accidentally allowed their custom email address to leak, although generally this isn’t a huge issue as emails which don’t match the user’s account/endpoints would be dropped and not send webmentions in any case. (In some sense it’s roughly equivalent to my being able to visit https://brid.gy/twitter/schnarfed and clicking on the
Poll now or
Crawl now buttons. It’s doable, but doesn’t give a bad actor much. You’d probably want to rate limit incoming emails to prevent against mass spam or DDoS sort of attacks against Bridgy.)
A side benefit of all of this is that those who have kept their old email notifications could relatively easily get much of their past missing back feed as well. Or if they’re missing back feed for some reason, they could easily get it by re-sending the relevant emails instead of some of the current manual methods. Perhaps allowing preformatted emails with those same manual methods could be used to do back feed for Facebook or other providers as well?
We could also put together some forwarding filters for common platforms like gmail to help people set up autoforwarders with appropriate keywords/data to cut down on the amount of false positive or password containing emails being sent to Bridgy.
The one potential privacy issue to consider(?) is that this set up may mean that Bridgy could be sending webmentions for private messages since users get both private and public message notifications whereas the API distinguished these in the past. To remedy this, the comment URL could be tested to see if/how it renders as a test for public/private prior to sending. Separately, since Bridgy doesn’t need to store or show these messages (for long?), private messages could be sent, but potentially with a payload that allows the receiving end to mark them as private (or to be moderated to use WordPress terminology). This would allow the user’s website to receive the notifications and give them the decision to show or not show them, though this may be a potential moral gray area as they could choose to show responses that the originator meant to be private communication. The API would have prevented this in the past, but this email method could potentially route around that.