This is best shown by example. Consider the network shown in Figure 3-1. A snads_local channel would connect the PMDF-XGS system naples.example.com with the SNADS world, handing off messages to the SNADS node BLUE, by way of the PMDF-XGS transport bridge bridge.example.com.

Figure 3-1 Sample SNADS and SMTP Site

On the PMDF side, one would add MX records for pseudodomain names such as blue.example.com, indigo.example.com, azule.example.com, and lapis.example.com pointing to the PMDF-XGS system naples.example.com.

On the SNADS side, one would at a minimum configure the SNADS nodes BLUE, INDIGO, AZULE, and LAPIS to believe that the PMDF-XGS transport bridge bridge.example.com is another SNADS node, and that naples.example.com is another SNADS node reachable through bridge.example.com. One would probably also configure the SNADS nodes to believe that milan is an additional SNADS nodes reachable through bridge.example.com.

That is, on the SMTP side, the SNADS nodes BLUE, INDIGO, AZULE, and LAPIS would appear to be additional SMTP systems blue.example.com, indigo.example.com, azule.example.com, and lapis.example.com reachable through naples.example.com; on the SNADS side, the SMTP systems naples.example.com and milan.example.com would appear to be additional SNADS nodes. NAPLES and MILAN would be reachable through the NAPLES transport bridge in its guise of SNADS node.

Then using the standard naming conventions, user carol at milan.example.com can address a user CHARLES at INDIGO as charles@indigo.example.com (or perhaps even as charles@indigo, if the TCP/IP package and PMDF system are configured to allow use of short form names). User BOB at AZULE can address mail to betty at naples.example.com as BETTY@NAPLES, but will not be able to address christopher at milan directly, because SNADS names are limited to eight characters. This is solved by sending the mail to the PMDF addressing channel by addressing the mail to MAILMAN@NAPLES, and embedding the addresses of the real recipients in the text of the note. This also applies to recipients at SMTP hosts for which PMDF has not been configured to convert the short form host name into a fully qualified domain name, or whose host name is over eight characters long. For instance a SNADS user could not directly address priscilla at prometheus, or susan@styx.hades.org.