sendmail's implementation features aliasing, forwarding, automatic routing to network gateways, and flexible configuration.
In a simple network, each node has an address, and resources can be identified with a host-resource pair. For example, a mail system can refer to users with a host-user-name pair. Station names and numbers must be administered by a central authority, but user names can be assigned locally to each station.
In an internetwork, multiple networks with different characteristics and management must communicate. In particular, the syntax and semantics of resource identification change. You can handle certain simple cases by using improvised techniques, such as providing network names that appear local to stations on other networks. However, the general case is extremely complex. For example, some networks require point-to-point routing, which simplifies the database update problem, because only adjacent stations are entered into the system tables; others use end-to-end addressing. Some networks use a left-associative syntax; others use a right-associative syntax, causing ambiguity in mixed addresses.
Internetwork standards seek to eliminate these problems. Initially, these standards proposed expanding the address pairs to address triples, consisting of network, station, resource. Network numbers must be universally agreed upon; stations can be assigned locally on each network. The user-level presentation was quickly expanded to address domains, composed of a local resource identification and a hierarchical domain specification with a common static root, as defined in RFC 1034. The domain technique separates the issue of physical versus logical addressing. For example, an address of the form "jane@iris1.company.com" describes only the logical organization of the address space.
sendmail bridges the gap between the world of totally isolated networks that know nothing of each other and the clean, tightly coupled world of unique network numbers. sendmail can accept old arbitrary address syntaxes, resolving ambiguities by using heuristics specified by the network administrator, as well as domain-based addressing. sendmail helps guide the conversion of message formats between disparate networks. In short, sendmail is designed to assist a graceful transition to consistent internetwork addressing schemes.