Simple Mail Transfer Protocol

SMTP stands for the Simple Mail Transmission Protocol and it is used fro the email transmission across the networks and internet.  Most of the Email servers use SMTP as their de factor standardized email communication protocol to send the emails from email client to the email server.  SMTP operates on the port 25 of the TCP/IP.  When you configure an email client such as outlook express, you have to configure the POP3, SMTP and IMAP.  

Email client program requires the IP address of the SMTP server in order to complete the configurations. SMTP is a text based protocol in which recipients of the message are specified along with the text of the message.   Today many mail servers support the Extended Simple Mail Transfer Protocol which allows multimedia files delivery.  When a client like Outlook Express sends the email message, it connects to the SMTP server (usually located at your web hosting company or local ISP) by using the SMTP protocol.

Sendmail is the mail transfer agent of the SMTP.  If the mail delivery fails the sendmail put the messages in queue and retry the delivery.  Some of the most popular SMTP server programs are Microsoft Exchange Server, Exim, Postfix and Novel Netmail.  SMTP protocols include the specific rules and the methods for relaying the messages and authenticating the users in order to prevent the abuses.

The relaying SMTP server determines that which SMTP server to connect by looking the MX DNS records for the recipients domain name that can be found at the right side of every email address.