For example, I regularly receive email for an individual who has a nearly identical email address to my own, but at ymail.com instead of gmail.com. Any system that tries to guess at ___domain misspellings is going to catch ymail.com and think "Ah ha, they meant to type gmail.com, I'll correct that for them!" Viola, their email is sent to me. Again, this is not a theory, it happens to this poor guy all the time.