The thing is, the handling of the local part is MTA specific: you cannot just assume that different local parts correspond to different people because of aliase

The thing is, the handling of the local part is MTA specific: you cannot just assume that different local parts correspond to different people because of aliases, +foo addresses and the fact that the administrator of the MTA can set whatever equivalence rules they like: I, for example, use - instead of + because so many internet forms reject + as a valid email character. The only sane way to handle this is email-based confirmation.