Message Transfer Agent - Transfer Versus Access

Transfer Versus Access

The function of an MTA is usually complemented with some means for email clients to access stored messages. This function typically employs a different protocol. The most widely implemented open protocols for the MUA are the Post Office Protocol (POP3) and the Internet Message Access Protocol (IMAP), but many proprietary systems exist (Exchange, Lotus Domino/Notes) for retrieving messages. Many systems also offer a web interface for reading and sending email that is independent of any particular MUA.

At its most basic, an MUA using POP3 downloads messages from the server mailbox onto the local computer for display in the MUA. Messages are generally removed from the server at the same time but most systems also allow a copy to be left behind as a backup. In contrast, an MUA using IMAP displays messages directly from the server, although a download option for archive purposes is usually also available. One advantage this gives IMAP is that the same messages are visible from any computer accessing the email account, since messages aren't routinely downloaded and deleted from the server. If set up properly, sent mail can be saved to the server also, in contrast with POP mail, where sent messages exist only in the local MUA and are not visible by other MUAs accessing the same account.

The IMAP protocol has features that allow uploading of mail messages and there are implementations that can be configured to also send messages like an MTA, which combine sending a copy and storing a copy in the Sent folder in one upload operation.

The reason for using SMTP as a standalone transfer protocol is twofold:

• To cope with discontinuous connections. Historically, inter-network connections were not continuously available as they are today and many readers didn't need an access protocol, as they could access their mailbox directly (as a file) through a terminal connection. SMTP, if configured to use backup MXes, can transparently cope with temporary local network outages. A message can be transmitted along a variable path by choosing the next hop from a preconfigured list of MXes with no intervention from the originating user.
• Submission policies. Modern systems are designed for users to submit messages to their local servers for policy, not technical, reasons. It was not always that way. For example, the original Eudora email client featured direct delivery of mail to the recipients' servers, out of necessity. Today, funneling email through MSA systems run by providers that in principle have some means of holding their users accountable for the generation of the email is a defense against spam and other forms of email abuse.