Put the auto rejects into its own list and let the user add it to an existing inbox.

So if I have have the inbox for Facebook.com email, then I notice unsorted messages coming from meta.com, I can merge this into the Facebook inbox. You can probably guess and recommend the correct merge because they’ll inbox in the To: line is already known.

I think if you’re centralizing the data for merging across all users, you can really get smart about recognizing or train an agent on that info.

I don’t think you should ever totally trash an email though. This will lose you users over time. Put them in a list/folder. Similar to how there are Junk and Spam folders where flagged emails get put. The user still gets to have those emails in case they need them or a false flag occurs.