Leaving a dryer sheet inside a mailbox is a well-known pest-repellent trick used by mail carriers and homeowners.

• Repelling Wasps & Yellow Jackets: Scented dryer sheets give off a strong aromatic scent that wasps, yellow jackets, and bees strongly dislike. Dark, enclosed metal or plastic mailboxes make prime locations for stings and nest-building during warmer months.
• Preventing Stings: Mail carriers frequently reach blindly into dozens of mailboxes daily. Placing a sheet at the back deters insects from setting up home inside, protecting both the mail carrier and the resident from getting stung when retrieving mail.
• Deterring Other Pests: The strong fragrance also helps deter other unwanted pests like ants, spiders, and silverfish.

So if you—or your tenant—notice dryer sheets in the mailbox, it’s best to leave them in place (and refresh them once they lose their scent)!