Well, just to throw a little fuel on the fire, the law is mixed as to whether the pursuit was legal. Here are some interesting citations.
Nelson v. Howell, 455 So.2d 608, 610 (Fla. Dist. Ct. App. 1984) (private citizen may arrest person who commits felony in his presence, and where citizen has such person in his custody and felon attempts to flee, citizen may use such force in preventing felon's escape "as is necessary, or as appears to him in the exercise of reasonable discretion to be necessary, even to the extent of taking life"); see also Collins v. State, 143 So.2d 700, 703 (Fla. Dist. Ct. App. 1962) (private citizen does have common law right to arrest person who commits felony in his presence)If in truth appellant's money had been taken from him by force and against his will, he, or any other private person, had a lawful right to pursue the thief for the purpose of arresting him, and of recapturing the money so taken. McGuff v. State, 248 Ala. 259, 261 (Ala. 1946)In carrying out such an arrest a private citizen has a right to pursue the felon and to use necessary force and any reasonable necessary means to effect that arrest or to prevent the escape of the person who has committed a felony in his presence, or whom he reasonably believes to be guilty of a felony, which has been committed. Smith v. State, 258 Ind. 594, 602 (Ind. 1972)
While each of the foregoing mentions felony, the turning factor seems to be the legality of the arrest. Most states allow for a citizen's arrest for a misdemeanor committed in the citizen's presence. Were there charges brought against the pursuer, he could well have a colorable claim of legal justification in defense.
Note, though, just to put up a defense to a criminal charge one could expect to spend at least $5k, a lot more than a mailbox. Also note, cops hate it when regular people seek to do their jobs.