IGNOU BCA 5th sem Solved Assignment - Explain the working of mail server specifying the protocols involved in sending and receiving the mails?

Explain the working of mail server specifying the protocols involved in sending and receiving the mails?  

Ans

Have you ever wondered what happens when you press the Sendbutton in your email program? Probably not, I guess — as long as it works. This is exactly the reason why you should wonder, though. In case something doesn't work it's good to know what does not work. Usually, that's half the solution.

When you send email SMTP comes into play. SMTP is short for Simple Mail Transfer Protocol as defined in RFC 2821: Simple Mail Transfer Protocol. Your mail client talks to the SMTP server using this clean and simple procedure to get email from one place to another.

The Flirt

Your email program becomes an SMTP client, connects to port 25 of your mail server (usually the SMTP port) and — says EHLO. Computers, in the end, are only human and what counts is that it wants to be polite. Actually, it does not attempt to be polite but to use later additions to SMTP that have brought about two flavors of the later HELO command (SMTP command generally consist of four characters).

Two Flavors of HELO

EHLO, being the more recent one makes the server advertise all the additional features (such as delivery status notification or the ability to transport messages that contain other than the safe ASCII characters) it supports.

Not every server will allow this greeting, but it is required to accept a plain HELO which naturally assumes that no additional features are present. Both hello commands do require the client to specify its domain after the **LO, however. In practice, this looks something like:

220 mail.domain.net ESMTP Server

HELO 501 HELO requires domain addressHELO localhost 250 mail.domain.net Hello localhost [127.0.0.1], pleased to meet you
250 sender@example.com... Sender ok
250 support@aboutguide.com... Recipient ok (will queue)

(My input is in italics, the servers output is black; lines beginning with a 5 indicate an error.)

The Sender

The remainder of the protocol really deserves the attribute simple. If you want to send an email, you start with the keywords MAIL FROM:. Following this comes the email address of the sender, as suggested by the from. Don't forget to put brackets around the address, though (like <sender@example.com>). Continuing our example, we have:

MAIL FROM: <sender@example.com>

The recipient

After the server has accepted the sender's address, the client can give the address of the recipient. The command for this action, RCPT TO: again is rather suggestive. I want to send mail to myself:

RCPT TO: recipient@example.com

That the server will queue means just that: it will save the mail locally and send it together with all the other queued mail in intervals (for example, every 30 minutes). This behavior depends on the configuration and the server may also deliver the mail instantly.

The Process of Sending an Email

Now that you know the basics about incoming and outgoing mail servers, it will be easier to understand the role that they play in the emailing process. The basic steps of this process are outlined below for your convenience.

Step #1: After composing a message and hitting send, your email client - whether it's Outlook Express or Gmail - connects to your domain's SMTP server. This server can be named many things; a standard example would be smtp.example.com.

Step #2: Your email client communicates with the SMTP server, giving it your email address, the recipient's email address, the message body and any attachments.

Step #3: The SMTP server processes the recipient's email address - especially its domain. If the domain name is the same as the sender's, the message is routed directly over to the domain's POP3 or IMAP server - no routing between servers is needed. If the domain is different, though, the SMTP server will have to communicate with the other domain's server.

Step #4: In order to find the recipient's server, the sender's SMTP server has to communicate with the DNS, or Domain Name Server. The DNS takes the recipient's email domain name and translates it into an IP address. The sender's SMTP server cannot route an email properly with a domain name alone; an IP address is a unique number that is assigned to every computer that is connected to the Internet. By knowing this information, an outgoing mail server can perform its work more efficiently.

Step #5: Now that the SMTP server has the recipient's IP address, it can connect to its SMTP server. This isn't usually done directly, though; instead, the message is routed along a series of unrelated SMTP servers until it arrives at its destination.

Step #6: The recipient's SMTP server scans the incoming message. If it recognizes the domain and the user name, it forwards the message along to the domain's POP3 or IMAP server. From there, it is placed in a sendmail queue until the recipient's email client allows it to be downloaded. At that point, the message can be read by the recipient.