In Texas, when a home is sold and the United States Postal Service owns the mailbox for delivery, the USPS wants their keys back! So what's an agent to do?
We can't ignore Paragraph 2C in the One To Four Family Residential Contract that lists mailbox keys as accessories that convey to the buyer. I'd never given this much thought, but the proper way to handle this is to note the keys in the following paragraph, 2D, as being an exclusion, to make sure the keys do not convey with the property.
Once the keys are turned in, the Post Office changes the locks and gives three new keys to the new homeowner. If the keys are conveyed directly to the new owner, those keys should be turned in to get new ones issued. You shouldn't need anything other than the closing documents and a government issued ID. There's no key deposit required.
The good news is that if the mailbox is privately owned, the sellers can just hand over the mailbox key at closing.
Cluster mailboxes, a centralized unit of individually locked compartments, could be owned by the subdivision or an HOA, so be sure to check.
You've got mail!