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

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

3. 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 messages.
Two types of MUAs are available: those that have a graphical user interface (GUI) and command line interface (CLI).

4. 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 message, the MUA sends it to a mail transfer agent (MTA), which transmits the message across the network.

5. 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 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.

6. 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).

7. 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

8. 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 ordinarily don’t use procmail.

9. 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 delivery occurs when the message recipient reads the message using her own MUA.