Mailboxes encountered in later levels may sometimes be a monster in disguise (starting around Level 5). Unlike regular mailboxes they cannot be used and when approached spring to life and chase you for a few seconds doing high damage. If you wait patiently they will peer out of the mail slot briefly and thus give themselves away.
The absolute safest tactic to use once you identify one is to use a boombox present to incapacitate them (one will always spawn on Level 1 [Fixed World]) and then follow up with some tomatoes. If you don't have a boombox the next best option is to use the Slingshot from distance. DO NOT use the standard tomatoes on their own as the throw distance is too short and leaves you vulnerable.
Another way to identify and attack them is to provoke them and then use either the edge of the map or water to block and safely wait out their rampaging attack. Once it's finished it will remain idle long enough for you to attack it. It takes a total of 10 tomatoes to defeat.