The Full form of SMTP is Simple Mail Transfer Protocol. SMTP is a standard protocol on an IP/TCP network for sending emails through servers. It is a part of the application layer of the IP/TCP protocol, which makes it possible to send email messages over the internet to one or more recipients. So, SMTP is used when an email is sent by an email user, such as Gmail or Outlook Express, to the recipient using email servers.

SMTP is maintained and developed by the IETF (Internet Engineering Task Force). It is also known as RFC 2821 and RFC 821. SMTP uses port 25 and it helps in setting up communication rules between servers and works by starting a session between server and the user, whereas Mail Delivery Agent (MDA) and Mail Transfer Agent (MTA) provide local delivery services, domain searching, respectively. The components of SMTP are: Mail User Agent (MUA), Mail Submission Agent (MSA), Mail Transfer Agent (MTA), and Mail Delivery Agent (MDA).

SMTP is a very reliable source of communication as it resend the email messages which are not sent successfully until the receiver of the email receives it. It is also very helpful in bulk email marketing in which the user can send emails to a large number of recipients at the same time.