I think you are a bit confused about the E-mail structure.
Everything behind the @ is the domain, on your case “domain.com” Before the @ is just a name that can be used as you, the domain owner, wants.
If you want to redirect all mail to yourname@domain.com, that’s very easy to do AND you can still see the original e-mail address these nails were sent to.
So I assume for example Dropbox sent some commercial mail about current offers. Using that, he knew the old account and that it was signed up to Dropbox
What if they decide tomorrow that critisising the government is unlawful? That you get arrested for being LGBTQ? Send “Free Palestine” at your friends and have cops at your door?
If they allow this scanning, they just need to edit a text file to immediately flag thousands for having certain beliefs.
Besides, the real criminals will just move encryption to a different level or find different ways to bypass this.
Ultimately, there’s only one solution for governments. That is to ban all encryption. And hopefully that’s something we never see happening.