Unit – 4 Chap - 2 Mail Delivery System Created by: Ashish Shah, J.M. Patel College of Commerce Mail Delivery System Unit – 4 Chap - 2

Tracing the Email Delivery Process Created by: Ashish Shah, J.M. Patel College of Commerce Tracing the Email Delivery Process In classic UNIX style, email creation, transmission, and delivery consist of several discrete tasks, each of which is handled by a program designed specifically for that task. The usual email delivery process involves three components, a mail user agent, a mail transfer agent, and a mail delivery agent.

Created by: Ashish Shah, J.M. Patel College of Commerce Mail User Agent (MUA) To be able to send mail, you or your users need a program called a mail user agent, commonly abbreviated as MUA and widely referred to as a mail client. The MUA provides users an interface for reading and writing email messages. Two types of MUAs are available: those that have a graphical user interface (GUI) and command line interface (CLI).

Created by: Ashish Shah, J.M. Patel College of Commerce MUA Whether your MUA is a GUI or CLI, the functionality is the same. After composing an email message, the MUA sends it to a mail transfer agent (MTA), which transmits the message across the network.

Mail Transfer Agent (MTA) Created by: Ashish Shah, J.M. Patel College of Commerce Mail Transfer Agent (MTA) A mail transfer agent, usually abbreviated as MTA, is the program that sends the message across the network. MTAs work without any intervention by the user. In fact, most users are unaware of the MTA; they just see their mail client. The MTA reads the information in the To: section of the email message and determines the IP address of the recipient’s mail server. Then the MTA tries to open a connection to the recipient’s server through a TCP/IP port, typically port 25.

Created by: Ashish Shah, J.M. Patel College of Commerce Role of DNS in MTA DNS, the Domain Name System, plays a role in mail deliver, too. MTAs query a domain’s DNS mail exchanger, or MX, record, to determine the host that is responsible for receiving mail for that domain. If the MTA on the sending machine can establish a connection to the destination MTA, it transmits the message using the Simple Message Transfer Protocol (SMTP).

Created by: Ashish Shah, J.M. Patel College of Commerce MTA In some cases, the message might not be immediately deliverable, so the sending MTAwill store in a local queue and attempt retransmission after a set amount of time. In other cases, the receiving MTAmight refuse delivery, causing the message to bounce and be returned to the sender’s MUA. Assuming that all goes well, the receiving MTA adds header information to the message. This header contains information for tracking the message and ensuring that it is delivered. Finally, the receiving MTApasses the message to yet another program, the mail delivery agent, for delivery to the recipient’s mailbox

Mail Delivery Agent (MDA) Created by: Ashish Shah, J.M. Patel College of Commerce Mail Delivery Agent (MDA) After the MDA (sometimes referred to as a local delivery agent, or LDA) receives the message from the MTA, it stores the new message in the recipient’s mailbox file. The mailbox file, known traditionally as the mail spool, is almost always identified by the recipient’s username. Ordinarily, the location of the user’s mailbox file is /var/spool/mail/username. In some cases, the user’s mailbox file will be a file or directory in the user’s home directory. Procmail isn’t the only possible MDA. Users who rely on POP or IMAP clients to retrieve their email ordinarily don’t use procmail.

Mail Delivery Agent (MDA) Created by: Ashish Shah, J.M. Patel College of Commerce Mail Delivery Agent (MDA) Rather the POP or IMAP client is itself the MDA. The next step in the process is optional. Some users (and many MUAs) provide some sort of visual or audible notification that new mail has arrived. The last step of email delivery occurs when the message recipient reads the message using her own MUA.