How does the international mail system work? How is mail routed from one country to another?

I’m mostly going to refer to the USPS below, because they put a great deal of their operating information online. However, you can be sure that even if the specifics differ, the principles apply to other postal operators just as well.

Domestic mail handling

International airmail is processed almost exactly the same as domestic mail on each end. This video from the USPS explains the collection/sorting/dispatch/delivery sequence pretty well. It also gives you a very good idea of what kind of handling your letters and parcels need to be able to stand up to.

International air mail routing

After collection and sorting, mail is dispatched to the appropriate International Service Center to catch an airplane.

As you might expect, the Post Office chooses flights for your parcel differently than you might choose flights for yourself. Chapter 5 of USPS handbook M-22 gives the best overview I’ve found of international air mail routing. A simplified overview:

The overall goal of routing is to choose the most cost-effective means possible that still meets the service standard. In other words, they choose the cheapest route that gets your mailpiece there by the promised time — not necessarily the fastest route. Moreover, the Post Office is also constrained by reliability requirements as well as political directives, as you’ll see below.

Frequency: air dispatches to a given country go at least 5 days a week so long as the following volume is met:

  • an average daily volume of letters, cards, small packets, etc, of 5kg or more, or
  • parcels numbering three or more, or weighing 5kg or more.

Priority: as explained above, when making up dispatches, higher priority items get loaded first. If an aircraft needs to unload mail in order to shed weight, lower priority items get removed first.

Flight scheduling and selection: regular flights are scheduled for mail airlift, based on their ability to meet the schedule. Alternates are selected as well. This means that the Postal Service usually has a specified amount of space booked on regular flights — they can’t just put mailpieces on any flight that’s headed in the right direction.

When it comes to selecting individual flights, US airlines get priority in selection, so long as they meet the service schedule. Flights are assessed as “best” to “worst” something like this:

  • single plane, nonstop
  • single plane, one stop
  • single plane, multiple stops
  • intraline service (single airline), including single flight designations that involve a change of aircraft
  • ramp-to-ramp service (guaranteed by the airline)
  • interline transfers
  • foreign flag carriers that offer usual USPS or UPU rates
  • US carriers to an intermediate country (which will then require a transit, as opposed to direct delivery to the destination country).

Preparing dispatches: the standards for how international mail dispatches are prepared can be found in the UPU Acts, specifically the Letter Post and Parcel Post volumes. Essentially:

  • items get packed into mailbags (or other such containers)
  • a manifest is drawn up and (usually) sent ahead to the destination post electronically
  • the dispatch is shipped off to the destination country’s designated postal facility for receiving international mail (Office Of Exchange)
  • the receiving postal adminstration unpacks the dispatch, verifies it against the manifest, presents items to customs, etc.

SAL and Surface mail

  • SAL is only dispatched in loads of 750lbs (~350kg) or more. This probably slows it down more than the surface transportation on each side of the air lift.
  • Similarly, mail going by sea probably needs to fill a shipping container before it goes.
  • Cargo ships usually make 15-20kt through the water, which means that (for example) a direct trip from Shanghai to Vancouver or San Francisco would be around 2-3 weeks. However, while they don’t really take circuitous routes per se, cargo ships often stop at many ports along the way to take on and offload cargo.
  • Much of your parcel’s time is likely spent waiting for the shipping container to fill, then waiting for a suitable ship to come pick it up.

References and further reading: