- It is normally transported by X-boats although subsidized merchants, tramp freighters, couriers, and many other kinds of vessels may take mail contracts.
- It is often exchanged via Postal Unions and Free Trade Zones.
- It is a kind of cargo often carried in Mail Vaults or secure Staterooms.
A variety of mail types exist from old-fashioned paper missives, to bulky packages, to crates of message tablets, to electronic messages, and more. Mail is typically carried within a secure Mail Vault by contract specification although mail might be transported otherwise when vaults are not available. A very wide variety of messaging and communications technologies exists throughout Charted Space. 
Despite the name X-mail or Express Mail, it can sometimes be quite slow, taking weeks, months, or even years to reach its destination. In the case of misjumps, some mail has even been recorded as taking decades or even centuries to reach its destination due to time dilation effects. And, of course, some mail NEVER reaches its destination. 
However, within the core sectors of the Third Imperium, X-Mail arrives within twice the minimum time between systems (...usually twice a week for many systems and several times a day in more frequented systems), along the X-Boat Routes, with 99.974% reliability. 
Mail Cargo is a special form of freight, typically consisting of large data storage drums which contain a vast amount of information that must be transported from one world to another, but is not vital enough to be entrusted to the X-Boat service or a private courier. 
The sheer size of the Third Imperium means that ordinary communications must depend on ships travelling along the main trade routes, making Regina (Regina / Spinward Marches 1910) nearly four years out from the Imperial Core Sector.
The express boat (abbreviated X-boat) system, established originally in 624 and expanded to cover the entire Imperium by 718, cuts this communication time by nearly 75 percent. Selected locations along major trade routes are established as sites for express stations, which are orbital facilities which service and refuel the X-boats on their communications runs. Often an express boat tender serves as the station.
The Xboat System was established by Empress Arbellatra in 624, and expanded to cover the entire Imperium by 718. It has been expanding to cover new areas of the Third Imperium ever since. Some political theorists think that the Imperium my have reached the practical limits of expansion possible for TL-15 communications and transportation technologies.
Primary X-mail Ship Types & Classes
- Type XT class Express Boat Tender