E-mail is delivered by having the sender establish a TCP connection to the receiver and then ship the e-mail over it. This model worked fine for decades when all ARPANET hosts were, in fact, on-line all the time to accept TCP connections.
However, with the advent of people who access the Internet by calling their ISP over a modem, it breaks down. The problem is this: what happens when Elinor wants to send Carolyn e-mail and Carolyn is not currently on-line? Elinor cannot establish a TCP connection to Carolyn and thus cannot run the SMTP protocol.
One solution is to have a message transfer agent on an ISP machine accept e-mail for its customers and store it in their mailboxes on an ISP machine. Since this agent can be on-line all the time, e-mail can be sent to it 24 hours a day.
POP3
Unfortunately, this solution creates another problem: how does the user get the e-mail from the ISP's message transfer agent? The solution to this problem is to create another protocol that allows user transfer agents (on client PCs) to contact the message transfer agent (on the ISP's machine) and allow e-mail to be copied from the ISP to the user.
One such protocol is POP3 (Post Office Protocol Version 3), which is described in RFC 1939. The situation that used to hold (both sender and receiver having a permanent connection to the Internet) is illustrated in Fig. 5-15(a). A situation in which the sender is (currently) on-line but the receiver is not is illustrated in Fig. 5-15(b).
Figure 5-15. (a) Sending and reading mail when the receiver has a permanent Internet connection and the user agent runs on the same machine as the message transfer agent. (b) Reading e-mail when the receiver has a dial-up connection to an ISP.
POP3 begins when the user starts the mail reader. The mail reader calls up the ISP (unless there is already a connection) and establishes a TCP connection with the message transfer agent at port 110. Once the connection has been established, the POP3 protocol goes through three states in sequence:
1. Authorization.
2. Transactions.
3. Update.
Figure 5-16. Using POP3 to fetch three messages.
Then the client can retrieve messages using the RETR command and mark them for deletion with DELE. When all messages have been retrieved (and possibly marked for deletion), the client gives the QUIT command to terminate the transaction state and enter the update state. When the server has deleted all the messages, it sends a reply and breaks the TCP connection.
In due course of time, Carolyn boots up her PC, connects to her ISP, and starts her e-mail program. The e-mail program establishes a TCP connection to the POP3 server at port 110 of the ISP's mail server machine.
The DNS name or IP address of this machine is typically configured when the e-mail program is installed or the subscription to the ISP is made. After the TCP connection has been established, Carolyn's e-mail program runs the POP3 protocol to fetch the contents of the mailbox to her hard disk using commands similar to those of Fig. 5-16.
Once all the e-mail has been transferred, the TCP connection is released. In fact, the connection to the ISP can also be broken now, since all the e-mail is on Carolyn's hard disk. Of course, to send a reply, the connection to the ISP will be needed again, so it is not generally broken right after fetching the e-mail.
IMAP
· For a user with one e-mail account at one ISP that is always accessed from one PC, POP3 works fine and is widely used due to its simplicity and robustness. For example, many people have a single e-mail account at work or school and want to access it from work, from their home PC, from their laptop when on business trips, and from cybercafes when on so-called vacation.
· While POP3 allows this, since it normally downloads all stored messages at each contact, the result is that the user's e-mail quickly gets spread over multiple machines, more or less at random, some of them not even the user's.
· This disadvantage gave rise to an alternative final delivery protocol, IMAP (Internet Message Access Protocol), which is defined in RFC 2060. Unlike POP3, which basically assumes that the user will clear out the mailbox on every contact and work off-line after that, IMAP assumes that all the e-mail will remain on the server indefinitely in multiple mailboxes.
· IMAP provides extensive mechanisms for reading messages or even parts of messages, a feature useful when using a slow modem to read the text part of a multipart message with large audio and video attachments.
· Since the working assumption is that messages will not be transferred to the user's computer for permanent storage, IMAP provides mechanisms for creating, destroying, and manipulating multiple mailboxes on the server.
Figure 5-17. A comparison of POP3 and IMAP
Delivery Features
Independently of whether POP3 or IMAP is used, many systems provide hooks for additional processing of incoming e-mail. An especially valuable feature for many e-mail users is the ability to set up filters.
These are rules that are checked when e-mail comes in or when the user agent is started. Each rule specifies a condition and an action. For example, a rule could say that any message received from the boss goes to mailbox number 1, any message from a select group of friends goes to mailbox number 2, and any message containing certain objectionable words in the Subject line is discarded without comment.
Some ISPs provide a filter that automatically categorizes incoming e-mail as either important or spam (junk e-mail) and stores each message in the corresponding mailbox. Such filters typically work by first checking to see if the source is a known spammer. If hundreds of users have just received a message with the same subject line, it is probably spam.
Webmail
Some Web sites, for example, Hotmail and Yahoo, provide e-mail service to anyone who wants it. They have normal message transfer agents listening to port 25 for incoming SMTP connections.
To contact, say, Hotmail, you have to acquire their DNS MX record, for example, by typing
on a UNIX system. Suppose that the mail server is called mx10.hotmail.com, then by typing
telnet mx10.hotmail.com 25
you can establish a TCP connection over which SMTP commands can be sent in the usual way.
When the user clicks on Sign In, the login name and password are sent to the server, which then validates them. If the login is successful, the server finds the user's mailbox and builds a listing similar to that of Fig. 5-8, only formatted as a Web page in HTML.
The Web page is then sent to the browser for display. Many of the items on the page are clickable, so messages can be read, deleted, and so on.
However, with the advent of people who access the Internet by calling their ISP over a modem, it breaks down. The problem is this: what happens when Elinor wants to send Carolyn e-mail and Carolyn is not currently on-line? Elinor cannot establish a TCP connection to Carolyn and thus cannot run the SMTP protocol.
One solution is to have a message transfer agent on an ISP machine accept e-mail for its customers and store it in their mailboxes on an ISP machine. Since this agent can be on-line all the time, e-mail can be sent to it 24 hours a day.
POP3
Unfortunately, this solution creates another problem: how does the user get the e-mail from the ISP's message transfer agent? The solution to this problem is to create another protocol that allows user transfer agents (on client PCs) to contact the message transfer agent (on the ISP's machine) and allow e-mail to be copied from the ISP to the user.
One such protocol is POP3 (Post Office Protocol Version 3), which is described in RFC 1939. The situation that used to hold (both sender and receiver having a permanent connection to the Internet) is illustrated in Fig. 5-15(a). A situation in which the sender is (currently) on-line but the receiver is not is illustrated in Fig. 5-15(b).
Figure 5-15. (a) Sending and reading mail when the receiver has a permanent Internet connection and the user agent runs on the same machine as the message transfer agent. (b) Reading e-mail when the receiver has a dial-up connection to an ISP.
POP3 begins when the user starts the mail reader. The mail reader calls up the ISP (unless there is already a connection) and establishes a TCP connection with the message transfer agent at port 110. Once the connection has been established, the POP3 protocol goes through three states in sequence:
1. Authorization.
2. Transactions.
3. Update.
Figure 5-16. Using POP3 to fetch three messages.
During the authorization state, the client sends over its user name and then its password. After a successful login, the client can then send over the LIST command, which causes the server to list the contents of the mailbox, one message per line, giving the length of that message. The list is terminated by a period.
Then the client can retrieve messages using the RETR command and mark them for deletion with DELE. When all messages have been retrieved (and possibly marked for deletion), the client gives the QUIT command to terminate the transaction state and enter the update state. When the server has deleted all the messages, it sends a reply and breaks the TCP connection.
In due course of time, Carolyn boots up her PC, connects to her ISP, and starts her e-mail program. The e-mail program establishes a TCP connection to the POP3 server at port 110 of the ISP's mail server machine.
The DNS name or IP address of this machine is typically configured when the e-mail program is installed or the subscription to the ISP is made. After the TCP connection has been established, Carolyn's e-mail program runs the POP3 protocol to fetch the contents of the mailbox to her hard disk using commands similar to those of Fig. 5-16.
Once all the e-mail has been transferred, the TCP connection is released. In fact, the connection to the ISP can also be broken now, since all the e-mail is on Carolyn's hard disk. Of course, to send a reply, the connection to the ISP will be needed again, so it is not generally broken right after fetching the e-mail.
IMAP
· For a user with one e-mail account at one ISP that is always accessed from one PC, POP3 works fine and is widely used due to its simplicity and robustness. For example, many people have a single e-mail account at work or school and want to access it from work, from their home PC, from their laptop when on business trips, and from cybercafes when on so-called vacation.
· While POP3 allows this, since it normally downloads all stored messages at each contact, the result is that the user's e-mail quickly gets spread over multiple machines, more or less at random, some of them not even the user's.
· This disadvantage gave rise to an alternative final delivery protocol, IMAP (Internet Message Access Protocol), which is defined in RFC 2060. Unlike POP3, which basically assumes that the user will clear out the mailbox on every contact and work off-line after that, IMAP assumes that all the e-mail will remain on the server indefinitely in multiple mailboxes.
· IMAP provides extensive mechanisms for reading messages or even parts of messages, a feature useful when using a slow modem to read the text part of a multipart message with large audio and video attachments.
· Since the working assumption is that messages will not be transferred to the user's computer for permanent storage, IMAP provides mechanisms for creating, destroying, and manipulating multiple mailboxes on the server.
Figure 5-17. A comparison of POP3 and IMAP
Independently of whether POP3 or IMAP is used, many systems provide hooks for additional processing of incoming e-mail. An especially valuable feature for many e-mail users is the ability to set up filters.
These are rules that are checked when e-mail comes in or when the user agent is started. Each rule specifies a condition and an action. For example, a rule could say that any message received from the boss goes to mailbox number 1, any message from a select group of friends goes to mailbox number 2, and any message containing certain objectionable words in the Subject line is discarded without comment.
Some ISPs provide a filter that automatically categorizes incoming e-mail as either important or spam (junk e-mail) and stores each message in the corresponding mailbox. Such filters typically work by first checking to see if the source is a known spammer. If hundreds of users have just received a message with the same subject line, it is probably spam.
Webmail
Some Web sites, for example, Hotmail and Yahoo, provide e-mail service to anyone who wants it. They have normal message transfer agents listening to port 25 for incoming SMTP connections.
To contact, say, Hotmail, you have to acquire their DNS MX record, for example, by typing
host –a –v hotmail.com
on a UNIX system. Suppose that the mail server is called mx10.hotmail.com, then by typing
telnet mx10.hotmail.com 25
you can establish a TCP connection over which SMTP commands can be sent in the usual way.
When the user clicks on Sign In, the login name and password are sent to the server, which then validates them. If the login is successful, the server finds the user's mailbox and builds a listing similar to that of Fig. 5-8, only formatted as a Web page in HTML.
The Web page is then sent to the browser for display. Many of the items on the page are clickable, so messages can be read, deleted, and so on.
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