Advanced Features Of SMTP Protocol

People send messages via electronic mail to each other by various methods on the mainframe computers until the SMTP protocol came. In August 1982 Information Sciences Institute proposed the standard method for sending and receiving emails which was called as the simple mail transfer protocol. Soon it became very popular on the Arpanet by replacing the older complicated methods for sending emails. Even though the protocol has been revised and extended regularly since that time the fundamental method for sending the mails remained unchanged. This protocol is a text-based protocol which originally does not support the delivery of binary data and however this made the protocol to be easy for the implementation and maintaining. Later multipurpose internet mail extensions (MIME) became popular in the 80s for encoding and sending the binary data via this protocol.  

Today SMTP remains the dominant protocol in the internet for sending and receiving emails. Since this protocol is text-based, learning it is very easy when compared with many others. All it requires to communicate with an SMTP server is a program capable of sending the ASCII data over the TCP/IP port 25. 

Email is sent via this protocol in a transaction i.e., if the process of sending the messages is not completed  in full then without generating any error the message is dropped. Transaction has three stages:

  • Specifying a sender, 
  • Provide one or more recipients, 
  • Send a message.

But SMTP transaction has five stages:

  • Handshake
  • Send reply address
  • Send recipients
  • Send  message data
  • End transaction


When connecting to Simple Mail Transfer Protocol SMTP Servers on port 25, the sender must wait for the receiver to accept the connection and identify itself like in the format:

220 [domain] [Service Information]

Then the sender must identify it to the receiver using the HELO command:

HELO [domain]

The mail servers will expect each to identify it using the domain name that could be used to verify the identity of the server by performing an MX record lookup. But the receiving server should not reject the connection during this stage even if the identity of the sender could not be verified and so should reply with:


 250 reply code from an SMTP server is the generic ok response to say the last action completed successfully.

Send reply address:

After the two servers have performed the handshake, the transaction has started and so the commands can be sent to the servers. Suppose if you want to know the commands supported by the server, then you can send the command HELP and the server will respond with a detailed message of the commands supported. To send a message to any user on this server, the reply address must be set first so that if any errors that occur during the transaction can be reported to this address, and for that MAIL FROM command is used. 

Send recipients:

After providing the reply address, SMTP should allow to identify the recipient(s) of the message which can be done by using the command RCPT TO. In order to set the multiple recipients you can simply repeat this command for each recipient. If the receiver accepts the mail for this user at this time, then it can be replied with a 250 ok reply. But if the mail is not accepted, a 550 failure reply or the appropriate error code will be sent. If a 250 Ok reply is received, then the process of sending the message data can be continued.

Send message data:

Sending the data through this protocol is simple but the message formats can be quite complicated particularly when sending binary attachments.  In order to start sending the data, DATA command must be sent to the server which must reply with a 354 intermediate reply. For sending the plain text messages, it is not required to use the multi-part MIME message format. All messages are ended by sending a line which contains only a period character. 

Ending the transaction:

QUIT command is sent for closing the connection and so the destination server will reply with a 221 reply to confirm the completion of the transaction. If the QUIT command is not sent before the closing of the connection, the destination server will then delete all the stored messages. 

What Is SMTP And How Does It Work

SMTP stands for Small Mail Transfer Protocol which is an internet standard for email transmission or electronic mail transmission. First, this particular protocol is defined by RFC 821 in the year 1982 and it was updated by RFC 5321 in the year 2008 with some additions. After this update only, this protocol is widely used by the several numbers of people around the world and through online tech blogs you can get more information about this topic.

Why using SMTP?

Even though email servers and some other mail transfer agenting platforms are using this SMTP protocol to send and also receive the mail messages, the client mail applications at the user level typically use the SMTP only to send the messages to the mail server especially for relaying. In order to receive the email messages, the client applications are usually using either POP3 or IMAP. When there is a SMTP communication between two or more mail servers, they are using the TCP port 25 protocol. When it comes to the mail clients, they often submit the outgoing electronic mails to the mail server on the port 465 or 587.

Although the proprietary mail systems such as IBM Notes & Microsoft Exchange and webmail systems such as Yahoo mail, Gmail and Outlook are using their very own non-standard protocols in order to access the account’s mail box on their own mail servers, all are using this SMTP protocol when receiving or sending electronic mail from outside their own systems. As email is emerging as the most valuable service on the web platform and the several numbers of internet systems are using this SMTP protocol, it has become too famous and highly demandable method for transferring the mail from one user to another. This specific email protocol generally used to send the mail through (POP) or IMAP are probably used to get back those mails at the end user or receiver side.

SMTP protocol model:

  • SMTP is actually a protocol on the application layer and the SMTP server is forever on the listening mode.
  • SMTP server will establish the TCP connection to send the mail across the recurrent connection instantly.
  • When considering the SMTP model, it is of two types such as,
  • End to end method
  • Store and forward method

If you are considering the end to end method in SMTP protocol, it is generally used to communicate between the various organizations whereas the store and forward method is used within a particular organization. If you are considering the SMTP client who wants to send an email, it will then contact the host of the SMTP destination directly for sending the email. The regarding server will carry on this mail to itself until it is effectively mock to the SMTP receiver. The user deals in such models with the user agent (UA) such as Mozilla, Netscape and Microsoft Outlook. For making the exchange of the mails, MTA and TCP are used in general.

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