Brittle Things: Email Architecture

One of the popular Internet services is electronic mail (e-mail). It is a process of exchanging digital messages from a person to one or more recipients. Here I will provide an overview of how email systems are organized and what they can do. The architecture of the email system is shown in the below figure,

Email Architecture

UA: user agent

MTA: message transfer agent

MAA: message access agent

The user agents allow people to read and send email. Some examples of user agents are Eudora, Outlook, and Netscape. Message transfer agent is used to pass message from the user agent to server (push message) and message access agent is used to access message from the server to the client system (pulls message). The process of sending messages into the mail system for delivery is called mail submission.

The e-mail systems use SMTP to send message from one server to another and to send message from client system to mail server. The messages can then be retrieved with an e-mail client using either POP or IMAP.

Simply SMTP is act as the mail transfer agent and it is used two times, between the sender and the sender's mail server and between the two mail servers. The two well known mail access protocols are IMAP and POP.

The POP has limited functionality. POP works on two modes: delete mode and keep mode. In the delete mode, the message is deleted from the mailbox after each retrieval. In the keep mode, the message remains in the mailbox after retrieval. Currently using version of POP is three (i.e., POP3). The second mail access protocol is IMAP (Internet Mail Access Protocol) version four (IMAP4). IMAP4 has more functionality than POP3 like, user can check email header before downloading, user can search for mail, user can create mailboxes on the mail server, and user can partially download email.

The traditional mail systems only allow text messages. In-order to send non-text data through mail the MIME (Multipurpose Internet Mail Extensions) is used. MIME transforms non-ASCII data at the sender system to NVT ASCII data and delivers them to the client MTA to send through the network. The system at the destination end transforms the NVT ASCII data to original data.