Simple Mail Transfer Protocol (SMTP)


Definition of Simple Mail Transfer Protocol (SMTP) in Network Encyclopedia.

What is Simple Mail Transfer Protocol (SMTP)?

Simple Mail Transfer Protocol is a standard application-layer protocol for delivery of e-mail over a TCP/IP internetwork such as the Internet. The basics of Simple Mail Transfer Protocol (SMTP) are defined in Request for Comments (RFC) 821 and 822.

Simple Mail Transfer Protocol
Simple Mail Transfer Protocol




How It Works

SMTP defines the format for messages sent between TCP/IP hosts on the Internet. SMTP uses plain 7-bit ASCII text to send e-mail messages and to issue SMTP commands to receiving hosts. Multipurpose Internet Mail Extensions (MIME) is typically used to encode multipart binary files including attachments into a form that SMTP can handle.



SMTP provides a mechanism for forwarding e-mail from one TCP/IP host to another over the Internet. SMTP services running on a TCP/IP host first establish a connection to a remote host using Transmission Control Protocol (TCP) port 25. An SMTP session is then initiated by sending a helo command and receiving an OK response. The sending computer then uses the following commands to send messages:

  • Mail fr: Identifies the sending host to the receiving host 
  • Rcpt to: Identifies the targeted message recipient to the receiving host by using the Domain Name System (DNS) format user@DNSdomain 
  • Data: Initiates the sending of the message body as a series of lines of ASCII text, ending with a single period (.) alone on a line 
  • Quit: Closes the SMTP connection 

Transport only between SMTP Hosts

SMTP provides message transport only from one SMTP host to another. Support for storing messages in mailboxes is provided by Post Office Protocol version 3 (POP3) and Internet Mail Access Protocol version 4 (IMAP4).

Implementing SMTP on a Microsoft Exchange Server

To implement SMTP on a Microsoft Exchange Server messaging network, install and configure the Internet Mail Service. To troubleshoot problems with remote SMTP servers, use Telnet to connect to port 25, try issuing various SMTP commands (such as the ones described in this entry), and examine the results. The SMTP service on Internet Information Server (IIS) version 4 functions only as an SMTP mail delivery agent (SMTP host) and does not have any provision for creating user mailboxes.

What is the Simple Mail Transfer Protocol (SMTP) and how it works?



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