Simple Mail Transfer Protocol

Simple Mail Transfer Protocol is considered as a part of TCP/IP protocol (Transmission Control Protocol/Internet Protocol) which moves your email across networks to the right email inbox and computer. SMTP uses the store and forward process with the Mail Transfer Agent (MTA) to move the emails across networks.

We will describe on how two networks works by exchanging the emails between the servers through SMTP process. For instance, Let us see the SMTP process that goes on if UserA sends an email from his server (Hotmail.com) to UserB’s email server (yahoo.com). Consider UserA using Hotmail as his email server and UserB using Yahoo mail server.

UserA’s email server (Hotmail) pushes the email to the destination UserB’s email server (Yahoo). The Hotmail email server starts communicating with the yahoo mail server for the mail deliver to happen. This is where the SMTP protocol comes up which governs the communication between the two mail servers (Hotmail email server and yahoo email server).

The SMTP protocol checking between the servers happens and then the Hotmail email server confirms the destination email address with the yahoo mail server for sending the email from UserA to UserB. After the confirmation, the Hotmail email server sends the email to the destination email address in the yahoo mail server.

SMTP Protocol: The Above SMTP protocol checking between the email servers is briefly explained below assuming that everything goes fine without any failure:

• The Hotmail server sends EHLO message to the yahoo server. The EHLO message is nothing but the greeting between the two servers. The yahoo server too sends the acknowledgement to the Hotmail server.

• After receiving the acknowledgement, the Hotmail server sends the destination address for confirmation. The receiving server (yahoo server) receives the message and sends acknowledgement after confirming the destination address.

• The Hotmail server then sends a message that it is going to send the email data after the destination address confirmation with the yahoo server.

• Once again after acknowledgement, the Hotmail server sends the email data to the yahoo server.

• The yahoo server will send an acknowledgement after receiving the full message and then both the servers will terminate the session.

The Internet service providers have a limit on the number of emails that can be sent over a certain amount of time. The limit is mostly set on the number of email sent per hour or per day. Each ISP relies on its SMTP to determine and govern the email that can be sent out by one connection. For some people who work at home or manage large mailing lists, that could be a problem. After they hit their limit, the ISP will simply stop sending emails. If the ISP thinks you're a spammer, they might even shut down your account.