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 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. 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.
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.
SMTP Functions
An SMTP server performs the following two functions :
1. It verifies the configuration and grants the permission to a computer that is making an attempt to send a message. It sends the message to the specified destination and tracks it to see whether it’s delivered successfully or not. If it’s not delivered successfully then an error message is send to the sender.
2. There’s one limitation to SMTP and it’s the inability to authenticate the senders and this results in e-mail spamming. The enhanced version of SMTP also exists and it’s called as Extended SMTP (ESMTP). ESMTP is used to send the e-mails that include graphics and other attachments.
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