Relay Agents

Most systems are configured to send all nonlocal outgoing E-Mail to a relay machine for delivery. This is done for two reasons:

• First, is simplifies the configuration of all MTAs other than the relay system's MTA.
• Second, it allows one system in an organization to act as the E-Mail hub, possibly hiding all the individual systems.

In our example the relay system has a hostname of mailhost in the local domain (.tuc.noao.edu) and all the individual systems are configured to send their E-Mail to this host. We can execute the host command to see how this name is defined to the DNS:

sun % host mailbox

mailhost.tuc.noao.edu CNAME noao.edu canonical name

noao.edu A 140.252.1.54 its real IP address

If the host used as the relay changes in the future, only the DNS name needs to be changed - the E-Mail configuration of all individual systems does not change.
Most organizations are using relay systems today.

The next figure is a picture of Internet E-Mail, taking into account that both the sending host and the final receiving host probably use a relay host. In this scenario there are four MTAs between the sender and the receiver.
The local MTA on the sender's host just delivers the E-Mail to its relay MTA. (This relay MTA could have a host name of mailhost in the organization's domain). This communication uses SMTP across the organization's local internet.
The relay MTA in the sender's organization then sends the E-Mail to the receiving organization's relay MTA across the Internet. This other relay MTA then delivers the E-Mail to the receiver's host, by communication with the local MTA on the receiver's host. All the MTAs in this example use SMTP, although the possibility exists for other protocols to be used.