Exam Prep: Learning Key for SMTP Protocol [Notes]


  • SMTP is the de-facto standard protocol for sending emails across the Internet
  • By default, the SMTP protocol works on three ports:

Port 25 – this is the default SMTP non-encrypted port

Port 2525 – this port is opened on all SiteGround servers in case port 25 is filtered (by your ISP for example) and you want to send non-encrypted emails with SMTP

Port 465 – this is the port used if you want to send messages using SMTP securely

  • SMTP operates at the Application Layer of the OSI model
  • Sendmail program released in the BSD operating system was the first mail transfer agents to implement SMTP.
  • SMTP is a delivery protocol and acts as a postman in delivering the mails to respective destination post-office.
  • Mail is routed based on the destination server, not the individual user(s) to which it is addressed.
  • Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for use by individual users for managing mail boxes by  retrieving messages
  • SMTP originated as a purely ASCII text-based, it did not deal well with binary files, or characters in many non-English languages. Standards such as Multipurpose Internet Mail Extensions (MIME) were developed to encode binary files for transfer through SMTP.
  • SMTP server access can be restricted by white-listing IP addresses and allowing access only if the accessing IP address belongs to the white-list. Alternatively SMTP servers may require authentication of clients by supplying valid credentials before allowing access.
  • The list of commans used for SMTP server communication are as follows:

HELO – introduce yourself

EHLO – introduce yourself and request extended mode

MAIL FROM: – specify the sender

RCPT TO: – specify the recipient

DATA – specify the body of the message (To, From and Subject should be the first three lines.)

RSET – reset

QUIT – quit the session

HELP – get help on commands

VRFY – verify an address

EXPN – expand an address

VERB – verbose


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