Just consider:
[log in to unmask][log in to unmask][log in to unmask]
(etc)
Same username. Same domain, except for the least significant part.
Totally different user. It would be a total disaster if mail software
were to consider these addresses to be the same. This is why the mail
standards say that these addresses are to be treated as totally different
people. For all you know, they aren't even in the same country!
Eric