Several categories of mail cannot be sent to a PO Box: packages too large for the box itself, shipments from private carriers like UPS and FedEx when addressed only to a box number, items that require your signature or a payment on delivery (these get held at the counter instead of placed in the box), legal service of process, and mail from banks, the DMV, and other institutions that need a verifiable physical address on file. A separate list of items — explosives, gasoline, marijuana, and others — cannot be mailed through USPS at all, whatever the destination.
Packages Too Large or Heavy for the Box
Every PO Box has a physical size limit. USPS offers five box sizes, from extra-small (3″ × 5½”) up to extra-large (22½” × 12″), and not every post office stocks every size.1USPS. PO Box Sizes Anything that doesn’t fit stays behind the counter, and the carrier leaves a PS Form 3849 redelivery notice inside your box telling you what arrived and where to pick it up.2USPS. PS Form 3849 Redelivery Notice
USPS also enforces a hard weight and dimension ceiling on every domestic package: 70 pounds maximum, and 130 inches of length and girth combined. Anything above either measurement is nonmailable and won’t be accepted into the system at all.3USPS. Parcel Size, Weight and Fee Standards So even a package small enough to fit through the counter is off the table if it exceeds the weight cap.
Packages Shipped by FedEx, UPS, DHL, and Amazon
Private carriers don’t have access to USPS facilities. FedEx, UPS, DHL, and Amazon’s own delivery drivers cannot open a PO Box. If someone ships you a package through one of these carriers addressed only to a PO Box number, the carrier will typically reject it or return it to the sender. This trips up a lot of online orders, since retailers often default to a non-USPS shipping method.
Street Addressing
Some post offices offer a service called Street Addressing that lets you use the post office’s physical street address combined with your box number (formatted as the street address followed by # and the box number). A private carrier reading a street address delivers to the post office building, and USPS staff place the item in your box or hold it at the counter.4USPS. PO Boxes Packages arriving this way still have to meet standard USPS rules, including the 70-pound limit.5Postal Explorer. 508 Recipient Services
Street Addressing isn’t offered everywhere. You activate it by signing a Customer Agreement for Premium PO Box Service enhancements at your local office and paying the applicable fee. Check availability when searching for a box on the USPS site or ask at the counter.
Hybrid Services
A few shipping options blend private-carrier and USPS handling. FedEx Ground Economy (formerly SmartPost) is the main one: FedEx handles the long-haul, then hands the package to USPS for the final delivery, which means it can reach a PO Box. UPS previously offered SurePost on a similar model but has moved those packages to its own drivers, so SurePost now requires a physical street address and won’t reach a PO Box through USPS.
Signature-Required and COD Mail
Certain USPS services require the recipient or an authorized agent to sign before the mail is released. A postal worker can’t collect a signature from an empty box, so these items never go inside it. You get a notification slip and pick up the item at the counter.
- Certified Mail: held for 15 days after the initial attempt
- Registered Mail: held for 15 days
- Insured Mail above certain value thresholds: held for 15 days
- Signature Confirmation and Adult Signature Required: held for 15 days
- Collect on Delivery (COD): held for 10 days, since payment is also owed before release6USPS. Collect on Delivery
Those hold periods are firm. Miss the 15-day window on Certified Mail and it goes back to the sender.7USPS. What Are the Second and Final Notice and Return Dates for Redelivery COD is tighter at 10 days.
Naming Someone to Pick Up for You
If you can’t reliably get to the counter, USPS lets you file a Standing Delivery Order using PS Form 3801. The form names one or more authorized agents who can sign for and collect Certified, Insured, COD, Registered, and Adult Signature Required mail on your behalf. Each agent must show a valid government-issued or employer-issued photo ID at the counter.8United States Postal Service (USPS). PS Form 3801 – Standing Delivery Order
Legal Service of Process
A PO Box is generally not a valid address for service of legal papers. Federal rules require a summons and complaint to be delivered in person, left at the defendant’s dwelling with a suitable person who lives there, or handed to an authorized agent.9Legal Information Institute (LII) at Cornell Law School. Rule 4 Summons A PO Box isn’t a dwelling, and postal workers aren’t authorized agents. Most state rules follow the same logic.
Some situations allow service by certified mail, which can technically reach a PO Box through the counter-pickup process described above. But when personal service is required, a PO Box won’t do it. If your PO Box is the only address you list on contracts, business filings, or correspondence, keep your physical address on file with any entity that might need to reach you for legal purposes.
Mail from Banks, the DMV, and Other Institutions Requiring a Physical Address
A number of government agencies and financial institutions won’t send certain mail to a PO Box because their own rules require a verifiable physical location on file. Common examples:
- Banks and financial institutions. Federal Know Your Customer and anti-money-laundering rules require a verifiable physical address, so account statements and official notices often can’t go to a PO Box.
- DMV and vehicle registration. Your driver’s license and vehicle registration generally have to be tied to a residential address.
- Business formation documents. Most states require a physical street address on incorporation or LLC filings.
- Jury duty notices. Courts use residential addresses drawn from voter registration or DMV records, not PO Boxes.
None of these restrictions come from USPS. The agencies and institutions set their own address requirements, but the effect is the same on your end: list only a PO Box and your application may be delayed or rejected, and important mail may never reach you.
Items USPS Won’t Carry to Any Address
Some things can’t be mailed through USPS at all, regardless of whether they’re headed to a PO Box, a street address, or a military APO. Domestically prohibited items include:
- Explosives and ammunition
- Gasoline and other flammable liquids
- Liquid mercury
- Marijuana, including for medical use (hemp and CBD products are allowed but restricted)
A separate category of restricted items can be mailed if you follow specific packaging and labeling rules. Aerosols, lithium batteries, alcoholic beverages, perishable foods, and live animals all fall here.10USPS. Shipping Restrictions and HAZMAT – What Can You Send in the Mail
Restricted items headed to a PO Box carry an added practical problem beyond the packaging rules: nobody is there to receive them immediately. USPS doesn’t guarantee timing for PO Box pickups, so a shipment of live day-old poultry or temperature-sensitive medication sitting in a box or at the counter for hours can be ruined. Perishable shipments to a box are especially risky, because the mailer assumes all the risk if the contents don’t arrive in time.