Since another post office enthusiast and photographic archivist -- indeed, a former Postmaster himself -- was recently harassed by a Postmaster in Missouri while taking a photograph of her post office (and given the experiences detailed in two previous posts), I feel compelled to specify what the laws (and postal regulations) say about post office photography.
On the issue of [exterior] post office photography:
So long as one is on public property, one may take photographs of whatever you please -- including federal buildings such as U.S. Post Offices. If that were prohibited, then Google Street View images would be blacked out around such facilities. You should see the high-resolution stuff they've got in Manhattan!
Nobody has the right to inspect any photographs you take from public property. The only exception is if law enforcement has probable cause to suspect that your photography is directly involved with some nefarious activity. (a.k.a. They have a specific reason to assume you're a terrorist.)
The only situation I can conceive for which that would apply to exterior postal photography is if one sits in the woods behind a post office, recording the activity of the trucks and the carriers for eight hours. THAT's kind of suspicious. Taking photos of the front? That's absolutely fine.
Photography policy: inside post offices...
Postal Operations Manual
§ 124.58 Photographs for News, Advertising, or Commercial Purposes
Photographs for news purposes may be taken in entrances, lobbies, foyers,
corridors, or auditoriums when used for public meetings, except as prohibited
by official signs, the directions of postal police officers, other authorized
personnel, or a federal court order or rule. Other photographs may be taken
only with the permission of the local postmaster or installation head.
(So yes, you can be restricted from taking photographs inside post offices, even just of murals, and even if it's just for archival purposes. If they do allow you to do so, they have the right to set whatever restrictions they so choose. Let's just hope they're friendlier and better informed than some of the places I've recently visited, then!)
P.S. In the case of the post office in Missouri, the Postmaster yelled at my friend, declared that his reason for taking a photo of the post office ("It's my hobby.") wasn't good enough, and copied down his license plate number to report to law enforcement. Of course, absolutely nothing became of that since she had no right to pull that garbage to begin with.