 FACT: When the mailman delivers mail to Ronnie Bostocks mailbox,he  raises the mailbox flag, presumably to signal to the resident that  mail has been delivered. (Ronnies girlfriend, seeing the mailbox  flag has been raised, seems to interpret the signal accordingly.)  Although it may be the convention for mail delivery wherever the  director/writer is from, it is not the case on Long Island, where it  is the custom for the resident to raise the mailbox flag to alert  the mailman that mail is in the mailbox waiting to be picked up.  Once the mail has been picked up, the mailman lowers the flag 