When given a message, smail first checks if the destination is the local host, or a remote site. If the target host address is one of the local hostnames configured in config, the message is handed to the director module. Otherwise, smail hands the destination address to a number of router drivers to find out which host to forward a message to. They can be described in the routers file; if this file does not exist, a set of default routers are used.
The destination host is passed to all routers in turn, and the one finding the most specific route is selected. Consider a message addressed to email@example.com. Then, one router might know a default route for all hosts in the bar.com domain, while another one has information for foo.bar.com itself. Since the latter is more specific, it is chosen over the former. If there are two routers that provide a ``best match'', the one coming first in the routers file is chosen.
This router now specifies the transport to be used, for instance UUCP, and generates a new destination address. The new address is passed to the transport along with the host to forward the message to. In the above example, smail might find out that foo.bar.com is to be reached via UUCP using the path ernie!bert. It will then generate a new target of bert!foo.bar.com!user, and have the UUCP transport use this as the envelope address to be passed to ernie.
When using the default setup, the following routers are available:
smail also recognizes IP-addresses written as dotted quad as a legal hostname, as long as they can be resolved through a gethostbyaddr(3) call. For example, firstname.lastname@example.org would be a valid although highly unusual mail address for scrooge on quark.physics.groucho.edu.
If your machine is on the Internet, these routers are not what you are looking for, because they do not support MX records. See below for what to do in this case.
These defaults work for many simple setups, but fail if routing requirements get a little more complicated. If you are faced with any of the problems discussed below, you will have to install your own routers file to override the defaults. A sample routers file you might start with is given in appendix-. Some distributions also come with a set of configuration files that are tailored to work around these difficulties.
Probably the worst problems arise when your host lives in a dual universe with both dial-up IP and UUCP links. You will then have hostnames in your hosts file that you only talk occasionally to through your SLIP link, so smail will attempt to deliver any mail for these hosts via SMTP. This is usually not what you want, because even if the SLIP link is activated regularly, SMTP is much slower than sending the mail over UUCP. With the default setup, there's no way escaping smail.
You can avoid this problem by having smail check the paths file before querying the resolver, and put all hosts you want to force UUCP delivery to into the paths file. If you don't want to send any messages over SMTP ever, you can also comment out the resolver-based routers altogether.
Another problem is that the default setup doesn't provide for true Internet mail routing, because the resolver-based router does not evaluate MX records. To enable full support for Internet mail routing, comment out this router, and uncomment the one that used BIND instead. There are, however, smail binaries included in some distributions that don't have BIND support compiled in. If you enable BIND, but get a message in the paniclog file saying ``router inet_hosts: driver bind not found'', then you have to get the sources and recompile smail (see section- above).
Finally, it is not generally a good idea to use the uuname driver. For one, it will generate a configuration error when you don't have UUCP installed, because no uuname command will be found. The second is when you have more sites listed in your UUCP Systems file than you actually have mail links with. These may be sites you only exchange news with, or sites you occasionally download files from via anonymous UUCP, but have no traffic with otherwise.
To work around the first problem, you can substitute a shell script for uuname which does a simple exit 0. The more general solution is, however, to edit the routers file and remove this driver altogether.