Domains and Policies

RFC 1006 describes domain-based naming. RFC 822 touches on this issue as well. Essentially, each host is given a name that is a right-to-left dot-qualified pseudo-path from a distinguished root. The elements of the path are organizational entities, not physical networks.

RFC 822 and RFC 976 specify how to parse certain sorts of addresses. You can configure sendmail to follow or ignore these rules.