Services » Mail (US Post Office)
Note: The nomenclature for these official US Post Office neighborhood mailboxes is "Cluster Box Units" or CBU.
Once a year, use Vaseline or another petroleum jelly to lubricate your key and gently insert, twist, and rotate several times. Also apply the lubricant to the back of the lock when the mailbox door is open. This will also often free a stuck or sticky lock set.