What is SMTP?

In today's information world, most communication is done through email. While you may know how to compile a message and send it or look at messages that have been sent to you, few are unfamiliar with the concept behind the technology itself.

SMTP is an acronym for Simple Mail Transport Protocol, and it is used as a network protocol to send email from one mail server to another across the internet. For example, when you create an email and send it, the email application software, such as Microsoft Outlook, has a setting that requires you to designate the outgoing mail server, which is normally provided by your Internet Service Provider's (ISP). This ISP's mail server transports the message on your behalf using SMTP to the recipient's ISP's mail server. It is the standard protocol that the majority of mail servers use.

How it works:

SMTP encodes a series of commands containing:

The SMTP mail server then transmits this information, which allows it to connect to the recipient's ISP's mail server. Once the connection is made, the receiving SMTP mail server verifies the information. If the information is valid, it grants permission to continue with the transfer. If the information is invalid, it is rejected, and a message is sent to the sender to indicate the reason why it was rejected. An invalid email address is one possible reason for the rejection.

The following is an example of a typical SMTP transfer between two servers: 

220 smtp.message.com ESMTP Postfix 
 HELO relay.company.org 
 250 Hello relay.company.org, I am glad to meet you 
 MAIL FROM:<bob@company.org> 
 250 Ok 
 RCPT TO:<alice@company.com> 
 250 Ok 
 DATA 
 354 End data with <CR><LF>.<CR><LF> 
 From: "Bob Smith" <bob@example.org> 
 To: Alice Brown <alice@example.com> 
 Cc: Fred@example.com 
 Date: Thurs., 6 Nov. 2008 08:15:43 
 Subject: meeting 
 Hello Mary. 
 Our meeting is at 2:00 p.m. today. 
 Your boss, 
 Gary 
 250 Ok: queued as 456789 
 QUIT 
 221 Bye
{The server closes the connection}

SMTP LIMITATIONS:

SMTP does not have the ability to authenticate the sender of the message, which results in users receiving annoying and unwanted spam email. However, the latest versions of most email application software and ISP providers have created various methods of detecting and tagging a majority of these unwanted nuances.

Warnings