Mail routing on UUCP networks is much more complicated than on the Internet, because the transport software does not perform any routing itself. In earlier times, all mail had to be addressed using bang paths. Bang paths specified a list of hosts through which to forward the message, separated by exclamation marks, and followed by the user's name. To address a letter to Janet User on a machine named moria, you would have used the path eek!swim!moria!janet. Whis would have sent the mail from your host to eek, from there on to swim and finally to moria.
The obvious drawback of this technique is that it requires you to remember much about the network topology, fast links, etc. Much worse than that, changes in the network topology -- like links being deleted or hosts being removed -- may cause messages to fail simply because you weren't aware of the change. And finally, in case you move to a different place, you will most likely have to update all these routes.
One thing, however, that made the use of source routing necessary was the presence of ambiguous hostnames: For instance, assume there are two sites named moria, one in the U.S., and one in France. Which site now does moria!janet refer to? This can be made clear by specifying what path to reach moria through.
The first step in disambiguating hostnames was the founding of The UUCP Mapping Project. It is located at Rutgers University, and registers all official UUCP hostnames, along with information on their UUCP neighbors and their geographic location, making sure no hostname is used twice. The information gathered by the Mapping Project is published as the Usenet Maps, which are distributed regularly through Usenet. A typical system entry in a Map (after removing the comments) looks like this.
moria bert(DAILY/2), swim(WEEKLY)
This entry says that moria has a link to bert, which it calls twice a day, and swim, which it calls weekly. We will come back to the Map file format in more detail below.
Using the connectivity information provided in the maps, you can automatically generate the full paths from your host to any destination site. This information is usually stored in the paths file, also called pathalias database sometimes. Assume the Maps state that you can reach bert through ernie, then a pathalias entry for moria generated from the Map snippet above may look like this:
If you now give a destination address of firstname.lastname@example.org, your MTA will pick the route shown above, and send the message to ernie with an envelope address of bert!moria!janet.
Building a paths file from the full Usenet maps is however not a very good idea. The information provided in them is usually rather distorted, and occasionally out of date. Therefore, only a number of major hosts use the complete UUCP world maps to build their paths file. Most sites only maintain routing information for sites in their neighborhood, and send any mail to sites they don't find in their databases to a smarter host with more complete routing information. This scheme is called smart-host routing. Hosts that have only one UUCP mail link (so-called leaf sites) don't do any routing of their own; they rely entirely on their smart-host.