Even though you may think that it’s sexist to use the term “mailman” instead of the gender-neutral terms “mailperson” or “letter carrier” the truth is that no matter what gender ratios the Postal Service uses for hiring quotas, much like the clownfish and other unusual coral reef-dwelling species, the staff that runs delivery routes for letters and packages undergoes a metamorphosis which slowly transforms all of them into the male gender.
The Postal Service denies any involvement, because the biological agents they force staff to drink is supposed to render their letter carriers into neutered, docile homonculi, not all men.