# Mailboxes & Slots A mailbox is defined less by its material than by which USPS-regulated install context it serves: a post by the curb, a box on the wall, or a slot through the door. Curbside and locking boxes need USPS's approval seal; wall boxes and slots mostly don't, and a lock can't be added later to a box not built for one. ## How it works USPS, not the manufacturer, decides what counts as a mailbox. A curbside box needs a Postmaster General (PMG) approval seal, because carriers must reach it from a moving vehicle without leaving the seat: that single rule, fixed at 41 to 45 inches high and 6 to 8 inches back from the curb since the Post Office standardized it, is why nearly every curbside box still uses the dome-shaped 'tunnel-top' silhouette the Post Office approved in 1915 and made mandatory in 1923. Wall-mount boxes skip the seal but must still hold a full day's mail. Door slots skip the box entirely and must meet a minimum opening so letters and flats pass flat. Locking versions of curbside and wall boxes, approved since 2001 to cut mail theft, add a one-way drop slot with its own size rule and can't be retrofitted onto a shell not built for one. [1] [8] | Family | What it is | Suits | |---|---|---| | Curbside post-mount | A large metal or plastic box on a post at the street, approved by USPS and built, in almost every case, around the 1915 dome-rectangular shape. | Rural and suburban homes with roadside delivery; the default US mailbox. | | Wall-mount | A smaller box fixed to the house near the front door, for walk-up carrier delivery; no USPS approval seal required. | Older urban and suburban homes, and anywhere curbside delivery isn't practical. | | Door slot | A hinged slot cut into or beside the front door itself; no box at all, just an opening and a flap. | Urban homes and older houses where the carrier walks to the door. | ## How an expert narrows the choice ```mermaid timeline section 1 · Mounting type curbside_post : ask Package capacity, Locking / theft-resistant, USPS approved (PMG seal), Material, Outgoing-mail signal flag : skip Slot orientation, Privacy hood wall_mount : ask Package capacity, Locking / theft-resistant, Material : skip Slot orientation, Privacy hood, USPS approved (PMG seal), Outgoing-mail signal flag door_slot : ask Slot orientation, Privacy hood : skip Package capacity, Locking / theft-resistant, USPS approved (PMG seal), Material, Capacity tier, Steel gauge (thickness), Incoming mail slot size, Outgoing-mail signal flag, Exterior dimensions section 2 · Package capacity yes : ask Capacity tier, Exterior dimensions no : skip Capacity tier section 3 · Locking / theft-resistant yes : ask Steel gauge (thickness), Incoming mail slot size no : skip Steel gauge (thickness), Incoming mail slot size ``` 1. **Mounting type.** Mail goes to a post, a wall box, or a door slot? Curbside, wall-mount and door-slot are separate USPS-regulated product classes with different approval rules, sizes and install specs; the answer removes two of the three families and most of their facets at once. If curbside_post: ask Package capacity, Locking / theft-resistant, USPS approved (PMG seal), Material, Outgoing-mail signal flag; skip Slot orientation, Privacy hood. If wall_mount: ask Package capacity, Locking / theft-resistant, Material; skip Slot orientation, Privacy hood, USPS approved (PMG seal), Outgoing-mail signal flag. If door_slot: ask Slot orientation, Privacy hood; skip Package capacity, Locking / theft-resistant, USPS approved (PMG seal), Material, Capacity tier, Steel gauge (thickness), Incoming mail slot size, Outgoing-mail signal flag, Exterior dimensions. Skip if already known: existing mailbox type, delivery address type. 2. **Package capacity.** Just letters and cards, or room for small packages too? Package-friendly boxes are a distinct USPS-named class (Next Generation Mailboxes) with a bigger, different-shaped opening; a letters-only answer keeps the buyer in the smaller, cheaper tier and skips the rest. If yes: ask Capacity tier, Exterior dimensions. If no: skip Capacity tier. Skip if already known: package delivery volume, an earlier capacity answer. 3. **Locking / theft-resistant.** Worried about mail or package theft? A lock can't be added later; a yes routes to a purpose-built model with its own steel gauge and incoming-slot spec, a no keeps the buyer in the larger decorative catalog. If yes: ask Steel gauge (thickness), Incoming mail slot size. If no: skip Steel gauge (thickness), Incoming mail slot size. Skip if already known: stated theft concern, prior lock purchase. ## What to surface ### Mounting type Whether the mailbox is a post-mounted box at the curb, a box on the wall, or a slot through the door. USPS treats these as three separate regulated classes, not three styles of the same thing. Only curbside boxes need the PMG approval seal; wall boxes skip it but must hold a day's mail; door slots skip the box and must meet their own minimum opening. Picking the wrong one for the house risks a carrier refusing delivery or a postmaster rejection, and switching from curbside to wall-mount needs the local postmaster's permission. [1] [3] Values: curbside_post, wall_mount, door_slot. Question to ask: Mail goes to a post, a wall box, or a door slot? Used as: top-level split, filter. Data: on the listing, backfill cost none. Look for it as: Stated plainly in the title ("curbside mailbox", "wall mount mailbox", "mail slot"). ### Package capacity Whether the box is sized for small packages and padded envelopes, not just letters and flats. USPS itself promotes a 'package-friendly' upgrade class (Next Generation Mailboxes) because standard boxes are too small for the package volume most households now get; the trade-off is a bigger, heavier box that may need a sturdier post. [2] [3] Question to ask: Just letters and cards, or room for small packages too? Used as: top-level split, filter. Data: derived from listing fields, backfill cost low. Look for it as: Inferred from stated interior dimensions or the word "parcel"/"package" in the title; rarely a labelled yes/no field. ### Locking / theft-resistant Whether the box has a purpose-built lock and a one-way drop slot, rather than a plain hinged door. USPS only approved locking curbside designs in 2001, specifically to cut mail and identity theft, and a lock can't be bolted onto a standard box afterward: locking models are a separate purpose-built product line with their own incoming-slot size and (usually) heavier steel. [8] [6] Question to ask: Worried about mail or package theft? Used as: top-level split, filter. Data: on the listing, backfill cost none. Look for it as: Stated plainly in the title ("locking mailbox") or as a bullet feature. ### Slot orientation Whether a door mail slot is cut horizontally or vertically. USPS hinges the two differently: a horizontal slot's flap must hinge at the top, a vertical slot must hinge on the side opposite the door's own hinges, so the choice isn't cosmetic, it sets which hardware fits a given door. [1] Values: horizontal, vertical. Question to ask: Horizontal slot, or vertical? Used as: filter. Data: on the listing, backfill cost none. Look for it as: Stated in the product title or orientation diagram. ### USPS approved (PMG seal) Whether the box carries the Postmaster General's seal of approval, required for curbside and locking mailboxes but not for wall-mount boxes. A curbside box without the seal can be refused by the local postmaster; this is a compliance gate, not a style preference, and it is the one place the governing standard shows up as a stocked fact on the product. Wall-mount boxes are exempt by USPS's own rule, so the facet only matters on two of the three mounting types. [1] [4] [6] Used as: filter. Data: on the listing, backfill cost low. Look for it as: Title or bullet states "USPS Approved"; the PMG seal is also stamped on the product and usually visible in listing photos. ### Capacity tier A brand's own small/medium/large/extra-large grouping by what the box can physically hold. This is where package capacity gets concrete: small holds envelopes and rolled magazines only, medium adds padded envelopes and small boxes, large adds shoe-box-sized parcels, extra large takes magazines laid flat plus large boxes. It is a merchandising convention, not a USPS size class, so the exact cutoff between tiers isn't published in inches. [3] Values: small, medium, large, extra_large. Question to ask: Small, medium, large, or extra-large capacity? Used as: filter, shown on the card, sortable. Data: needs research, backfill cost medium. Look for it as: Architectural Mailboxes groups its own product line this way on its site; most general retail listings don't carry a labelled capacity-tier field. Backfill from stated interior dimensions where given, or from the brand's own product-line placement for known brands; otherwise leave null rather than guess. Chart: ordinal position, small to extra-large. ### Material What the box body is made of: steel, aluminum, or polypropylene plastic. Steel is the heaviest and the material almost every locking, pry-resistant design uses, with a powder coat to stop rust; aluminum won't rust at all and is lighter, which is why coastal and salt-road areas favour it; plastic never rusts or dents visibly but offers the least resistance to prying and is mostly a decorative-box material. [3] [8] Values: steel, aluminum, plastic_polymer. Question to ask: Steel, aluminum, or plastic? Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Material line in the title or bullets. Chart: color by material. ### Privacy hood An inside flap over a door mail slot that blocks the view into the house through the opening. A plain slot lets someone see or reach into the entryway; a hood closes that gap, and USPS caps how far it can project (about 2 1/16 inches) so it doesn't jam the carrier's delivery. [1] Question to ask: Want a privacy hood over the slot? Used as: filter, shown on the card. Data: on the listing, backfill cost none. Look for it as: Listed as an included accessory or add-on option. ### Outgoing-mail signal flag Whether the box includes a flag that flips up to tell the carrier there's outgoing mail to pick up. Carriers only collect posted mail when they can see the flag raised from the road; a box missing or without one loses the outgoing-mail option until it's fixed, so this is a carrier-workflow feature, not decoration. [4] [6] Question to ask: Want an outgoing-mail signal flag? Used as: filter, shown on the card. Data: on the listing, backfill cost none. Look for it as: Stated in the title/bullets, or sold separately as a replacement flag kit. ### Incoming mail slot size The minimum opening a locking mailbox's one-way drop slot must meet so flats and Priority Mail envelopes pass without creasing. USPS requires at least 1.75 by 10 inches so an unfolded Priority Mail flat-rate envelope fits; a narrower slot forces the carrier to hold mail and leave a notice, which can trigger a delivery suspension if it keeps happening. [8] [6] Used as: shown on the card. Data: needs research, backfill cost medium. Rarely a labelled field; found on the maker's own spec page for most purpose-built locking models. ### Steel gauge (thickness) How thick the steel body is, in millimetres; a lower gauge number means thicker steel. This is the real pry-resistance number on a locking mailbox, but almost no decorative box publishes it, and the gauge figure means nothing without a reference point: one leading brand's flagship model is 2.0 to 2.5mm (12 to 14 gauge), while a retailer describes similar units as 14 to 16 gauge, so the number only becomes useful compared against a known security model, not read alone. [4] [7] Range: 1.6 to 2.5 mm. 1.6-2.5mm (12-16 gauge) spans two sources disagreeing about one flagship locking model, not an industry-wide figure: the maker states 12-14 gauge, a reseller describes similar units as 14-16 gauge. Decorative, non-locking boxes are thinner but brands rarely publish a number for them. Used as: shown on the card, sortable. Data: needs research, backfill cost medium. Only on the maker's own product page for security-branded models; leave null for decorative boxes rather than assume a thin gauge. Chart: Steel gauge / pry-resistance scale. ### Exterior dimensions The box's own outer width, height and depth, as stated on the maker's product or spec page. This is the number that makes the package-friendly trade-off checkable against a real product: a bigger box can outgrow an existing post or crowd the curb, but that claim is only as good as a stated footprint, not a guess. [4] [2] Used as: shown on the card. Data: needs research, backfill cost medium. Look for it as: Rarely a labelled field on general listings; found on the maker's own product/spec page. Backfill from the maker's own product page; most general retail listings don't carry a full W x H x D figure. ## What you give up - One leading maker's flagship locking mailbox runs about 38 lb of 12-to-16-gauge steel, versus a few pounds of thin steel or plastic for a decorative box, costs more, and can't be added onto an existing non-locking shell later. [4] Board branch: "See more lightweight, decorative mailboxes". - A package-friendly box is deeper and heavier than a standard one, so it can outgrow an existing post and becomes a bigger object at the curb; brands sell matched mailbox-and-post kits for exactly this reason. Board branch: "See more standard-size mailboxes". ## How to draw it **USPS placement diagram** (other, shows Mounting type). Shows the 41-to-45-inch height and 6-to-8-inch curb setback every curbside box must meet; the one diagram USPS and every brand's install guide repeats. Used today by USPS; repeated in Architectural Mailboxes' and Mail Boss's own installation guides. [1] [5] **Steel gauge / pry-resistance scale** (scale, shows Steel gauge (thickness)). Places a box's steel gauge against known security models, since a gauge number means nothing without a reference point to compare it to. This is a proposal, nobody draws it this way yet. [4] ## Words you will meet - **PMG seal**. A mark on an approved mailbox showing it meets USPS's size and construction rules, named for the Postmaster General. - **Next Generation Mailbox**. USPS's name for a package-friendly curbside mailbox upgrade, sized for small parcels as well as letters. - **Breakaway post**. A mailbox post designed to collapse on vehicle impact instead of staying rigid, so it doesn't injure the driver. - **Mail fishing**. Reaching into a mailbox's slot by hand or with a tool to pull mail out, which a one-way drop slot is designed to stop. - **Steel gauge**. A number scale for sheet-metal thickness where a lower number means thicker steel. - **Tunnel-top**. The arched, dome-shaped roof on most curbside mailboxes, from the 1915 design USPS still bases its approved shape on. ## Where we made a call - USPS's curbside design rules, not material or styling, are what actually standardizes this category. (confidence high). Four independent sources (USPS, Architectural, Mail Boss twice) state the same 41-45in/6-8in figures. [1] [3] [5] [6] - The current engineering standard's letter is unclear: STD-7B in older material, STD-7C in a 2026 guide. (confidence medium). Sources disagree. Could not fetch the primary USPS engineering document to confirm which revision is current; neither name is asserted as fact in any facet. [6] - A mailbox post's breakaway design is a vehicle-occupant safety rule, not a postal one. (confidence high). Sourced to the Federal Highway Administration via USPS's own installation page. [1] [6] - Theft is the stated reason locking mailboxes exist, but household-level theft-rate data was not found. (confidence medium). The only theft-rate study found covers centralized cluster boxes, not single-household curbside theft. [8] - Capacity tiers are a brand convention, not a USPS size class, despite reading like one. (confidence medium). Architectural's own tier page gives no inch or cubic-foot thresholds; treat as approximate, not regulated. [3] ## References 1. [How to Set Up & Mount a Mailbox](https://www.usps.com/manage/mailboxes.htm) (regulator). Supports: 41-45in height and 6-8in setback for curbside boxes; PMG seal requirement; wall-mount boxes exempt; door slot opening, height and hinge rules; breakaway post materials and burial depth. 2. [The Next Generation Mailboxes](https://www.usps.com/packagemailbox/) (regulator). Supports: package-friendly mailbox is a named USPS upgrade class for curbside boxes; example package-friendly interior dimensions. 3. [Residential Mailbox Guide](https://web.archive.org/web/2024/https://www.architecturalmailboxes.com/mailbox-buying-guide/) (spec sheet). Supports: small/medium/large/extra-large capacity tier content lists; steel, aluminum and plastic material trade-offs; second source for 41-45in/6-8in placement figures; curbside, wall-mount and mail-slot as three distinct product lines. 4. [Mail Boss locking curbside mailbox, product page](https://mailboss.com/products/mailboss) (spec sheet). Supports: 38lb, 2.0-2.5mm (12-14 gauge) galvanized steel for a flagship locking model; incoming package slot opening and mail-fishing deterrence; USPS-approved claim on a locking model. 5. [Mail Boss installation guide](https://mailboss.myshopify.com/pages/installation-guide) (spec sheet). Supports: third independent source for 41-45in/6-8in placement; locking boxes measure height at the incoming slot, not the box bottom. 6. [USPS Mailbox Requirements: The Complete Compliance Guide for 2026](https://www.budgetmailboxes.com/blog/usps-mailboxes-a-builders-guide-to-compliance) (primer). Supports: fourth source for 41-45in/6-8in placement; second source for 1.5x7in door slot and 1.75x10in locking-slot minimums; locks cannot be retrofitted onto non-locking designs; breakaway post is an FHWA vehicle-safety rule; cluster/4C boxes are a separate builder-purchased, multi-family standard. 7. [USPS Approved Mailboxes](https://www.budgetmailboxes.com/usps-approved-mailboxes.html) (retail listing). Supports: a second, differing gauge figure (14-16 gauge) for locking steel mailboxes. 8. [Letter box](https://en.wikipedia.org/wiki/Letter_box) (primer). Supports: Traditional/Contemporary/Locking USPS classification and No.1-3 sizes; second source for 1.75x10in locking-slot minimum; 1915 Joroleman design and 1923 mandatory mailbox history; locking curbside mailboxes approved by USPS since 2001; steel, aluminum or polymer plastic as locking-mailbox materials.