What a mailbox post is allowed to be.

USPS asks one thing of a post, and it is not a dimension: “The post or other support for a curbside mailbox must be neat and of adequate strength and size” (DMM 508.3.2.4, Domestic Mail Manual 508 (retrieved September 22, 2026)). The same manual adds that “the post or other support is not part of the receptacle”. Everything specific comes from the road side instead, because the post stands in the road’s right-of-way.

In Virginia that is VDOT, and the reassuring half first: an ordinary conforming mailbox needs no permission. The regulation says mailboxes “may be placed within VDOT right-of-way without a permit; however, placement shall not interfere with safety, maintenance, or use of the roadway” (24VAC30-151-560 (retrieved September 22, 2026)).

Four more lines from the same sheet, each of which rules out something people commonly do:

• “Mailbox posts shall not be set in concrete unless the design has been shown to meet VDOT breakaway standards.”
• “A metal post should not be fitted with an anchor plate; however, an anti-twist device that extends no more than 10 inches below the ground surface is acceptable.” (Again with the inch mark spelled out.)
• “The post-to-box attachment should be of sufficient strength to prevent the box from separating from the post top if a vehicle strikes the installation.”
• On a curb and gutter street, no part of the box or its supporting structure may overhang the road surface, gutter included, or interfere with pedestrian traffic.

The federal drawing behind that thinking is FHWA’s, and its numbers are not identical to VDOT’s, which is worth knowing if you have read both. FHWA’s standard drawing W 646-1 says posts “may be 4 x 4 inch or 4 inch diameter wood posts or 1.5 to 2 inch diameter standard steel or aluminum pipe posts embedded not more than 24 inches in the ground”, and that supports “shall conform to the requirements of the AASHTO Manual for Assessing Safety Hardware (MASH) or NCHRP Report 350” (FHWA, WFL Standard Drawing W 646-1 (retrieved September 22, 2026)).

On round wood the gap has closed, and the guidance sheet is the one that is out of date. Virginia’s regulation was amended effective 19 November 2025 and now reads: “Breakaway structures are defined as a single four-inch by four-inch square or four-inch diameter wooden post or a standard strength, metal pipe post with no greater than a two-inch diameter” (24VAC30-151-560, as amended 2025-11-19 (retrieved September 22, 2026)). So round wood is four inches in the regulation and four inches at FHWA — the 4½ inch figure appears only in the undated VDOT guidance sheet quoted above, whose own file metadata dates it to 2019. Where the two disagree the regulation is what governs, so build to four inches. Pipe is the one place they still differ: up to two inches in the Virginia regulation, 1.5 to 2 inches on the federal drawing, and two inches satisfies both.

It is widely repeated that a brick or stone mailbox surround is against the rules. We went looking for the rule and there is no federal or Virginia prohibition to find. FHWA has in fact accepted a proprietary simulated stone mailbox column for use on the National Highway System after crash testing, on the strength of it breaking apart at the base on impact. A real brick pillar is a different object from a hollow shell designed to do that, but the blanket claim is not one we can support.

What is true in Virginia is more specific and more useful. A masonry surround is a non-conforming structure in VDOT’s language. Its guidance sheet says non-conforming mailbox structures “may remain in-place under the auspices of a VDOT land use permit”, sets a minimum distance from the curb face for them, and says a structure “deemed as a safety hazard should be removed immediately from the highway right-of-way”. So it is not banned. It is a permit, a setback, and a structure VDOT may require you to take out.

Wood is the default here and it fails in one place: the few inches either side of the ground line, where soil, water and air meet. The USDA Forest Products Laboratory’s Wood Handbook draws the line that matters, and it is not the line most people think cedar sits on:

In that handbook’s four-class table, the cedars, white oak and old-growth redwood are resistant, which is the above-ground class. Black locust is very resistant, the ground-contact one. Douglas-fir and young-growth redwood are moderately resistant. So a cedar post is a good above-ground post and is not, on this reading, a ground-contact post — which is the opposite of how cedar is usually sold. The same chapter warns that decay resistance “may be less for members placed in contact with the ground” and to use caution with naturally durable woods in ground contact.

The other route is pressure treatment, and treated is not one thing. The American Wood Protection Association’s use categories separate “above ground exposed” from “ground contact general use” (AWPA Standard U1-25, excerpt (retrieved September 22, 2026)), and AWPA’s own homeowner sheet puts posts squarely in the second: “POSTS / GROUND CONTACT / Use Category 4A or higher” (AWPA residential use guide (retrieved September 22, 2026)). Lumber stamped for above-ground use, buried, is the most common way a wooden mailbox post dies early.

A standard-strength 2 inch steel pipe is on VDOT’s acceptable list, as is any steel or aluminum shape of equal or lesser strength. Two details from the same sheet are easy to get wrong: no anchor plate, and an anti-twist device no deeper than 10 inches. Both exist so the post moves when it is hit rather than stopping a car.

Metal’s failure mode is corrosion rather than rot, and it starts where the coating is broken, which is usually a drill hole, a cut end or the ground line. The rusty post guide covers telling a cosmetic problem from a structural one.

Vinyl is the honest awkward case. It is not on VDOT’s list of acceptable supports, and that list is written by enumeration — wood, steel, aluminum — so vinyl is left out rather than named and refused. We found no Virginia or federal document prohibiting a vinyl mailbox post, and we also found no crash-test acceptance for one. Absent from a list is a weaker fact than banned, and we are not going to upgrade it.

On the material itself there is one good primary source. NIST’s study of weathered PVC reports that “in tensile tests, U-PVC samples showed a shift from ductile to brittle behavior with increasing UV exposure”, that weathering causes “loss of mechanical properties, yellowing, and chalking”, and that dark PVC reaching over 70 degrees Celsius may degrade thermally as well as photochemically (NIST Technical Note 2304 (retrieved September 22, 2026)). A black vinyl post in a Virginia July is the case that describes.

And the sentence that settles how much to expect from a vinyl post: the ASTM specification that governs PVC fence and railing profiles says of itself that “loadbearing characteristics for fence and railing assemblies are not addressed within this specification” (ASTM F964-13(2019) (retrieved September 22, 2026)). Most vinyl posts are a sleeve over something else. It is worth knowing what the something else is.

The folklore that vinyl shatters in cold weather we could not support. The nearest evaluation we found covers PVC rail systems from minus 20 to 125 degrees Fahrenheit, which does not read as a Northern Virginia winter problem.

Not for wood, not for galvanized steel, not for vinyl, not for paint. Every year-count we found traces back to a trade body or a content site with no published method behind it, so there is nothing here to quote. What the sources do support is where each material fails, which is more useful when you are standing in front of one.