A letter mailed in one country and delivered in another is handled by at least two separate postal operators. The arrangement letting a stamp bought in one place buy delivery in another is treaty-based.
A treaty framework binds the operators
National postal operators participate in an international union with a common set of rules covering formats, handling obligations and the settlement of accounts between members.
The core principle is that mail properly posted in one member country will be treated as domestic mail once it reaches the destination operator.
Without that, every operator would need a bilateral agreement with every other, and international mail would be priced and routed far less uniformly than it is.
Terminal dues settle the accounting
The destination operator performs final delivery but sold no postage, so a system of payments compensates it for handling inbound mail from other members.
Rates depend on categories and on the classification of the countries involved, and they have been revised repeatedly as mail volumes and cost structures changed.
Disputes about whether these rates reflect actual delivery costs have driven significant reform, particularly as lightweight parcels from e-commerce grew as a share of the flow.
Exchange offices are the handover points
Outbound international mail is consolidated at designated exchange offices, sorted by destination country, containerized and sent by air or surface transport.
On arrival, the receiving operator's exchange office breaks down the containers and injects the contents into its domestic sorting network.
This is the stage where tracking often goes quiet, since a handover between two operators' systems is a seam where scan data may not carry across.
Customs sits inside the flow
Items containing goods require a customs declaration, and the destination country's authorities decide whether duty or tax applies and whether contents are admissible.
Postal operators act as intermediaries here, presenting items for inspection and collecting charges on behalf of the authorities before release for delivery.
Delays attributed to the mail are frequently delays at this stage, because clearance depends on the customs authority rather than on the postal operator's own capacity.
Private carriers run a parallel system
Express companies operate their own networks end to end, with their own aircraft, clearance arrangements and tracking, rather than handing off between national operators.
That integration is what buys speed and continuous visibility, and it is also why the pricing sits well above ordinary international post.
The two systems increasingly overlap, with postal operators offering tracked and expedited products that borrow features from the express model while remaining inside the treaty framework.