Simple Mail Transfer Protocol (SMTP) is a specific protocol designed to deliver email. Based on the TCP/IP suite of Internet protocols, it comprises communication guidelines that mail user agents (desktop email clients or webmail services) such as Gmail, Yahoo, Outlook, etc. use to send mail to an email server (SMTP server). TCP/IP is a protocol used by computing devices to connect to the Internet. The SMTP protocol is also used by mail servers to interact with each other or when a mail has to be sent from the server on the sender’s side (SMTP server) to the server on the recipient’s side. All email clients support SMTP. However, the SMTP servers that various mail clients use may differ.
SMTP was first created during the early 1980s. The protocol was originally defined in Request for Comments (RFC) 788, which was published in November, 1981. It replaced mail transfer protocol (MTP), which was restricted to FTP-defined capabilities. This meant MTP couldn’t possibly accommodate features particular to receiving and sending email.
SMTP was specifically designed for transporting electronic email. It’s a protocol running independently over TCP. Developments and refinement of SMTP continued over the years. In August, 1982, RFC 821 and RFC 822 were released. RFC 821 reworked SMTP, which became the protocol’s defining standard.
A port is essentially the route through which email protocols such as SMTP and Post Office Protocol (POP) connect to an email client. There are different SMTP ports but the two most important are port 25 and port 587. When SMTP was first introduced, port 25 was allocated to it by the Internet Assigned Numbers Authority (IANA) in RFC 821 document. However, port 25 is blocked by more than a handful of modern ISPs, thanks to its unsecured nature that allows individuals to spread malware and spam. That said, port 25 is still not completely out. It’s predominantly used to relay messages between email servers.
The submitting aspect, which is sending email to a mail server from a mail client, is taken care of by port 587. In other words, email client such as Apple Mail or Outlook invariably use port 587 to send email messages to the server. Port 587 is therefore called email submission port. Unlike port 25, port 587 is more secure since it authenticates every email that heads to the mail server. Port 587 has more or less become the default port for modern email systems.
Some other SMTP ports are port 465 and port 2525. Port 465 is not used for SMTP anymore but it still is identified with SMTP due to the brief period it was associated with SMTPS, SMTP’s encrypted version. Port 2525 isn’t recognized or sanctioned by IANA or IETF (Internet Engineering Task Force). However, port 2525 is still used as an alternative SMTP port to port 587. For instance, if other ports are blocked or there are connectivity issues, port 2525 may come to the rescue.