MUAs, MTAs, and MDAs

An electronic mail system can be broken down into three elements: the Mail User Agent (MUA), the Mail Transport Agent (MTA), and the Mail Delivery Agent (MDA).

The MUA is the user interface—the software with which the user reads his mail, organizes mail into directories or folders, and sends mail. People prefer different features in their MUAs, and not all MUAs are available on all platforms. Many MUAs can coexist on the same machine. For example, a UNIX workstation can have any or all of the following MUAs available for use: mailx, elm, pine, mutt, mailtool, and dtmail. A given user can use any MUA present on his or her system because the MUAs are simply local applications. In addition, MUA functionality is often included in multipurpose software such as Lotus Notes and Netscape Mail.

The MTA isn’t used to write a mail message; it’s used to route the mail from a local MUA to another MTA on another system. (sendmail is an example of an MTA, which is not used to read or write mail directly. sendmail is intended only to deliver preformatted messages.) Mail routing may occur either locally or remotely. In a local mail transfer in which both the sender and destination have accounts on the same machine, the MTA is responsible for transporting mail from itself to a local MDA. In the process, the MTA may possibly edit the protocols, addresses, and routing of the mail message. A message created on a UUCP network requires some transformation before that message can be received by a person on a TCP/IP network. The MTA acts as a gateway for mail to get a message from one network to another network that uses different protocols. In the vast majority of situations, there will be only a single MTA on a given machine.

The MDA is the third component of the mail handling routine. While sendmail handles SMTP mail transfer between MTAs directly, sendmail relies on Mail Delivery Agents (MDAs) to handle local delivery from the sendmail queue to a queue used by an MUA. Two common MDAs that sendmail is often configured to use are /bin/mail and procmail. /bin/mail is almost universally available on UNIX systems; procmail is widely available and is both faster and much more capable than the standard /bin/mail, providing strong capabilities for the presorting and preprocessing of mail.

To better understand the MUA/MTA/MDA relationship, consider a real-world example of a person sending a letter. The MUA represents the person sending the letter: He writes a letter, places it in an envelope, puts an address and stamp on it, and then delivers it to a post office. The MTA is like the post office staff: They accept the letter, examine the address, reformat the address if necessary, and route the letter either to a mailbox in the same post office (if the letter is local) or to another post office (for a remote destination). The MDA corresponds to the postal worker who delivers the mail from the post office to the intended location. If a gateway is used, this analogy can be extended: An MTA that receives a letter for a destination in another state has to transfer that message to another MTA that knows how to deliver letters in the target state.

The IETF Requests for Comment

An RFC is a formal description of protocol formats used on the Internet. These protocols are also adhered to by many non-Internet systems. The Requests for Comment (RFC) are issued by the Internet Engineering Task Force (IETF). RFCs are identified and referred to by numbers for clarity; it’s easier to refer to RFC822 than to refer to the “Standard for the Format of ARPA Internet Text Messages.” There are more than two thousand RFCs as of this writing, some of which have been made obsolete by later RFCs. To find a given RFC, look for the IETF on the World Wide Web at http://www.ietf.org/.

Because mail is such a commonly used function of the Internet, many of the RFCs set standards for mail exchange. sendmail and other MTAs address the needs and definitions of many of these protocols. However, attempting to describe in detail all of the RFCs relevant to mail transport and format could take years of time and thousands of pages.