If the app/site requires a user to click a verification link before using an email address, how the mail provider handles the local part is irrelevant. The use

If the app/site requires a user to click a verification link before using an email address, how the mail provider handles the local part is irrelevant. The user can either access the email because they used an address they "own" or they can't. It also solves a whole bunch of other issues all at once (typos, unsolicited signups, etc).