When it’s time to replace mailboxes in an existing building, we always recommend upgrading to either 4C mailboxes or cluster mailboxes (as shown above). However, we understand that sometimes this may not be possible due to wall depths, wall widths, wall materials, or available space for outdoor delivery. In this case, there are other options for replacement and retrofit installations. The 4B mailboxes below are USPS approved only if being used to replace the same style existing mailboxes. This should not affect the existing rough opening of the wall and the wall should not be modified to accept the new 4B mailboxes. For major renovations, 4C mailboxes or cluster mailboxes are required.