Drivers must keep the area surrounding a mailbox clear of all obstacles. If a mailman cannot reasonably approach the mailbox in his vehicle, he can opt not to deliver mail on that day.
The U.S. Postal Service doesn't specify exactly how close a driver can park to a mailbox. However, it's against USPS rules to obstruct a mailbox, so drivers should not park immediately in front or beside one. In many jurisdictions, the law requires mail recipients to maintain a curb-side mailbox that mailmen can reach without leaving their vehicles. In this case, even if the mailbox is easily accessible by foot, the postmaster may still deem it inaccessible.