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