The MTA is a powerful and complicated set of modules. Because the MTA represents the primary functionality of the Messaging Server system, it is described in detail in this section. An MTA can be used to coordinate message transfer between a small number of computers on a local network or to orchestrate the transfer of thousands of messages beyond the local network to millions of Internet users. The Messaging Server's primary message channel is the SMTP channel, which formats messages according to the open standard employed on the Internet. Local mail and incoming messages for users with local accounts are delivered by the local delivery channel.
Handlers help route messages.
Usually, incoming messages are sent to the account handler, which determines to whom the message should be delivered. These three basic components of the Messaging Server MTA are shown in Figure A.2 and are discussed in greater detail in the following sections. Message channels, the local delivery channel, and handlers. Message channels The Messaging Server message channels exchange messages with other MTAs and accept new messages from mail clients. Message channels can relay messages to and from other the Messaging Server modules and, in some cases, accept and dispatch messages without consulting other modules.