Programming Internet Email

Transferring Mail by MTAs

Mail transfer agents do not transfer mail. They make transfer decisions about messages and hand them off to mail delivery agents for the actual transfer. In other words, MTAs are responsible for ensuring that a message is sent to its next destination (not necessarily its final one) and that the message is of the appropriate form to get there and be properly interpreted.

MTAs receive messages from either MUAs or other MTAs. They parse information provided by the SMTP protocol called envelope information to determine the message’s sender and recipient. They rewrite the message as needed to pass through their part of the network, then they pass the message to a mail delivery agent for delivery to a local mailbox or transfer to another MTA or mail system.

Envelope information is part of the SMTP protocol and separate from the message itself. A full description of the SMTP envelope is given in Chapter 9.

Part or all of a message may need to be rewritten by an MTA en route. This could take place because the message is destined to another type of network. Different networks and protocols allow different types of data to be passed. In the case of Internet mail, only simple US-ASCII text characters are allowed in mail messages. Binary data is encoded in that character set. Other networks may be more or less restrictive. If a message is received by an MTA destined for the Internet, the MTA is responsible for ensuring that the message is properly encoded and will cleanly ...