Introduction to the technology: What is Email protocol? The email protocol is a method used to transmit or exchange information between email servers and clients. The three most familiar protocols are SMTP, POP and MIME. Diagram 1: SMTP only can be used to send an email to the mail server, and receive the email by using POP. SMTP Full name of SMTP is Simple Mail Transfer Protocol. The SMTP protocol only can be used to send emails to a mail server from a mail client, not to receive emails. The most important functions those perform by SMTP are User verification and Delivery failed Notification. SMTP verifies whether the sender who 's accessing SMTP server has the authorization to send an email message, so we must have valid authorization such as SMTP username and password to access it. Furthermore, SMTP delivers the email messages to the recipients and verify whether the email messages are delivered successfully. If the email messages cannot be delivered. It will pull back and send an error message to the sender. Advantages of SMTP: Reliability If the email is not successfully delivered, the SMTP server will try to resend the email until the transmission is successful. If the email really cannot be delivered to the recipient, the SMPT server will pull back and send an error message back to sender. It 's simple SMTP is the simplest method and eases to deliver an email between various clients in a network. Users just need to type and click send to the recipient 's