In the past people had a hard time trying to communicate with those who are miles away from them and for them to get in touch they used to write letters with no guarantee it will reach to the receiver but in case it does it will take days maybe months. As the time passed and technology has developed, something called Email has been created. According to Walter Ulrich (1980), E-mail is a way of forwarding a message content electronically and Its document is physically transmitted to its destination using transportation methods. Meaning that the contents of the messages is transformed into electrical signals and forwarded over communication channels for at least some portion of its journey. E-mail has been developed in conjunction with the Advanced Research Projects Agency Network (ARPANET) in the late 1960s and early ’70s. ARPANET was the precursor to the modern internet that was built as a research project by the US Department of Defence. Original emails were text only, did not support having documents, photos or video attached to them, or even different fonts. It was only in the 1980s when ARPANET morphed into the current internet that email started to look like it does today (“Intro To E-mail”, n.d.).
Generally, E-mail gives people the ability to send or receive messages through communication networks such as the internet regardless of the network being worldwide or local. As anything else in this world, email has a system. Palme (1995) believes that many of the electronic mail systems are already connected together in networks, so that users can send mail to each other, regardless of which mail system each of them is connected to.He also thinks that In the future, almost all systems will be connected in this way. This means that all the electronic mail systems, when connected, behave as one large system. This large system may eventually be comparable in size and complexity to the world-wide international telephone network, but will have more advanced technical functions, and will be more of a data-processing system than the telephone network(Palme, 1995).
According to Riabov (2005), SMTP is used as the common mechanism for transporting electronic mail among different hosts within the transmission control protocol/Internet protocol (TCP/IP) suite. It is an application layer protocol. Under SMTP, a client SMTP process opens a TCP connection to a server SMTP process on a remote host and attempts to send mail across the connection.
An SMTP-based process can transfer electronic mail to another process on the same network or to another network via a relay or gateway process accessible to both networks (Sheldon, 2001). An e-mail message may pass through a number of intermediate relay or gateway hosts on its path from a sender to a recipient. A simple model of the components of the SMTP system is shown in Figure 1(Riabov, 2005).
Users deal with a user agent (UA). The exchange of mail using TCP is performed by an MTA. Users normally do not deal with the MTA. It is the responsibility of the system administrator to set up the local MTA. Users often have a choice, however, for their user agent (Stevens, 1993). The MTA maintains a mail queue so that it can schedule repeat delivery attempts in case a remote server is unable. Also the local MTA delivers mail to mailboxes, and the information can be downloaded by the UA (see Figure 1).
As a result of a user mail request, the sender-SMTP establishes a two-way connection with a receiver-SMTP. The receiver-SMTP can be either the ultimate destination or an intermediate one (known as a mail gateway). The sender-SMTP will generate commands, which are replied to by the receiver-SMTP (see Figure 1). Both the SMTP client and server should have two basic components: UA and local MTA. There are few cases of sending electronic-mail messages across networks. In the first case of communication between the sender and the receiver across the network (see Figure 1), the sender’s UA prepares the message, creates the envelope, and puts message in the envelope. The MTA transfers the mail across the network to the TCP-port 25 of the receiver’s MTA. In the second case of communication between the sending host (client) and the receiving host (server), relaying could be involved (see Figure 2). In addition to one MTA at the sender site and one at the receiving site, other MTAs, acting as client or server, can relay the electronic mail across the network.
The system of relays allows sites that do not use the TCP/IP protocol suite to send electronic mail to users on other sites that may or may not use the TCP/IP protocol suite. This third scenario of communication between the sender and the receiver can be accomplished through the use of an e-mail gateway, which is a relay MTA that can receive electronic mail prepared by a protocol other than SMTP and transform it to the SMTP format before sending it. The e-mail gateway can also receive electronic mail in the SMTP format, change it to another format, and then send it to the MTA of the client that does not use the TCP/IP protocol suite (Forouzan, 2003). In various implementations, there is the capability to exchange mail between the TCP/IP SMTP mailing system and the locally used mailing systems. These applications are called mail gateways or mail bridges. Sending mail through a mail gateway may alter the end-to-end delivery specification, because SMTP will only guarantee delivery to the mail-gateway host, not to the real destination host, which is located beyond the TCP/IP network. When a mail gate- way is used, the SMTP end-to-end transmission is host-to- gateway, gateway-to-host or gateway-to-gateway; the behavior beyond the gateway is not defined by SMTP (Riabov, 2005). The Post Office Protocol version 3 (POP3), as defined in RFC 1939, is an “offline” protocol designed to provide the Internet community with a tool for connecting to mail servers and retrieving email messages. Since the Simple Mail Transfer Protocol (SMTP) functions only as a transport protocol between mail servers, it is not capable of delivering mail from the post office to a user’s mailbox. It also does not allow remote users to get mail from the server. Thus, a delivery protocol like POP3 is needed so that the mail received by the server from the Internet can be delivered to their intended recipients (POP3 and Internet Electronic Mail, 1998).
Riabov (2005) mentioned that POP3 start preforming once the electronic mail arrives from the network and is placed in the user’s inbox on the server. POP3 is designed so that mail client software can determine which messages have been previously downloaded from the server. The mail client can then download only new messages. Also, it works in a very clear and an organized way where its session goes through a number of states during its lifetime (see Figure 1). After TCP connection has been established and the POP3 server has sent a greeting, the session enters the AUTHORIZATION state. While in this state, the client identifies itself to the POP3 server. After this stage, the server acquires resources associated with the client’s maildrop. The session then enters the TRANSACTION state, wherein the client requests actions on the part of the server. Once the client has issued the QUIT command, the POP3 server releases all the resources acquired during the TRANSACTION state and says goodbye. The TCP connection is then terminated(POP3 and Internet Electronic Mail, 1998).
When a client gives an unrecognized, unimplemented or syntactically invalid command, the server automatically responds with a negative status indicator. The same response is given when a command is issued in an incorrect state. There is no general method for a client to distinguish between a server that does not implement an optional command and another server that is unable or unwilling to process the command.
A POP3 server may have an inactivity auto logout timer. Such a timer must be set to a duration of at least 10 minutes. If the server receives a command from the client during that interval, the auto logout timer is reset. When the timer expires, the session is prevented from entering the UPDATE state, that is the server terminates the TCP connection without removing any messages or sending any response to the client.
IMAP is a protocol for retrieving e-mail messages (RFC 1064). The IMAP4 version is similar to POP3 but supports some additional features. For example, with IMAP4, the user can search through his or her e-mail messages for key words while the messages are still on the mail server. The user can then choose which messages to download to his or her machine. IMAP uses SMTP as its transport mechanism. Following the simple analogy (Sheldon, 2001), IMAP servers are like post offices, whereas SMTP is like the postal carriers. IMAP uses TCP to take advantage of its reliable data deliv- ery services, which are allocated on the TCP port 143. The latest IMAP version 4, revision 1 (IMAP4rev1) is defined in RFC 2060.
IMAP has many advanced features, such as the ability to address mail not by arrival number, but by us- ing attributes (e.g., “Download the latest message from Smith”). This feature allows the mailbox to be structured more like a relational database system rather than a sequence of messages (Tanenbaum, 2003).