So Which Entries are Really Required?

When not using any of the optional dbm tables, sendmail+IDA delivers mail via the DEFAULT_MAILER (and possibly RELAY_HOST and RELAY_MAILER) defined in the sendmail.m4 file used to generate sendmail.cf. It is easily possible to override this behavior through entries in the domaintable or uucpxtable.

A generic site that is on Internet and speaks Domain Name Service, or one that is UUCP-only and forwards all mail via UUCP through a smart RELAY_HOST, probably does not need any specific table entries at all.

Virtually all systems should set the DEFAULT_HOST and PSEUDONYMS macros, which define the canonical site name and aliases it is known by, and DEFAULT_MAILER. If all you have is a relay host and relay mailer, you don't need to set these defaults since it works automagically.

UUCP hosts will probably also need to set UUCPNAME to their official UUCP name. They will also probably set RELAY_MAILER, and RELAY_HOST which enable smart-host routing through a mail relay. The mail transport to be used is defined in RELAY_MAILER and should usually be UUCP-A for UUCP sites.

If your site is SMTP-only and talks `Domain Name Service', you would change the DEFAULT_MAILER to TCP-A and probably delete the RELAY_MAILER and RELAY_HOST lines.