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Simple Mail Transfer Protocol (SMTP)
SImple Mail Transfer Protocol is an Internet protocol designed to send and receive e-mail messages between e-mail servers over the Internet.

SMTP was developed to send e-mail messages across the Internet. In the OSI model, SMPT is an application layer protocol that utilizes TCP as the transport protocol to transmit mail to a destination mail exchanger, in other words, SMTP is used to transmit mail to a mail server. Mail can be transmitted by a client to the mail exchanger server, or from mail exchanger to mail exchanger. Mail sent via SMTP is usually sent from one mail exchanger to another, directly. E-mail was never designed to be instantaneous, but that is often how it appears to us.

Mail Exchangers (MX)

Mail Exchangers are the name given to the applications that support the SMTP protocol. Mail Exchangers such as sendmail or Microsoft Exchange should listen for IP datagrams that arrive on the network interface with a TCP port number of 25 and on . This port is one of the 'well known ports' defined in RFC 1700. When a message is received, the mail exchanger should check to see if it is for one of it's users, then move the mail to the user's mailbox.

To identify the mail exchangers for a domain name, DNS zone files for the domain contain an MX resource record identifying the host name and IP address of the mail exchangers.

Simple mail transfer protocol differs from Post Office Protocol version 3 (POP3). POP3 is used by e-mail client applications such as Microsoft Outlook, Mozilla Thunderbird, Eudora and other e-mail applications to retreive mail stored in personal mailboxes at the mail server.

E-Mail Clients

E-mail is a client-server protocol that allows the exchange of messages and attachments in various formats. An e-mail client is a software application which seamlessly handles all the technical communications tasks to connect to and find the e-mail at the server, download the e-mail messages, organizes them and presents them to the user in a usable format. An e-mail client also provides the means to compose new messages, reply to and forward received messages, and to organize the messages for later review.

E-mail clients use POP3 or IMAP instead of SMTP. SMTP is used strictly between mail servers.

  • Outlook Express
  • Outlook
  • Mozilla Thunderbird

Web Mail Systems

Web mail systems provide a website that handles all the tasks normally reserved for a local e-mail application. Users can open a web browser and connect to Google, Yahoo, AOL and several other providers and perform most if not all of the tasks that can be performed with a standard e-mail client.

  • Gmail
  • Yahoo Mail
  • AOL Mails


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