SMTP stands for Simple Mail Transfer Protocol. It is the standard protocol for sending emails over the Internet. This protocol may use a number of ports to facilitate a connection between your email client and server. We’ll explain how ports work and which one you should use.
What Is A Port?
A port is a unique number used to distinguish between multiple applications sending data to the same internet destination (IP address) simultaneously. Imagine your computer is an apartment building. The IP address is the building’s street address, but in order to deliver a parcel to the intended recipient, you also need to know the room number (which is, in our case, the port number).
For most applications, including SMTP, the port numbers are standardized.
A Brief History Of SMTP Ports
SMTP was developed by computer scientists and researchers in the 1980s. It was built to meet the need for a standard protocol for exchanging emails instead of the mash-up of different protocols used in the Internet’s early days.
Port 25 is the standard port for communicating between SMTP servers. Nowadays many internet service providers block incoming traffic from customers on port 25 due to anti-spam concerns; for end users, however, this is rarely an issue.
In the 1980s, port 587 was established as the default for communication between an email client and server. This port is still in use for SMTP today.
Port 465, another submission port, is peculiar; it was never formally recognized as a transmission port by the Internet Engineering Task Force (IETF), the organization overseeing the SMTP protocol. This port was set aside for SMTPS, a more secure version of the protocol, but revoked after a year of existence in favor of another system. Yet, most internet service providers and email providers still support it.
Likewise, 2525 is not formally recognized, but you can still use it to transmit emails using SMTP.
Which Port Should You Use?
The most widely used ports are described below. In addition, SMTP providers may use their own port numbers for authenticated connections; to learn more about that, you’ll need to contact your provider.
This SMTP port number is the most common one for relaying email. However, as it’s very common, spammers and malicious actors often try to abuse it to send dodgy messages. Most internet service providers and cloud hosting providers therefore block this port to prevent spam.
This number has been reassigned for a different purpose by the Internet Assigned Numbers Authority (IANA), the organization overseeing the global IP address system. It’s no longer assigned for SMTP communications, but many legacy systems still use it. You can use this port if your SMTP application allows it, but we don’t recommend this if other options are available.
This is the default port number for transmitting emails. It will usually be the best option when one needs to route a message from one server to another. Combining this port with Transport Layer Security (TLS) encryption means your message will transmit securely, with no interception. All mail server operators should consider using port 587.
2525 is not officially recognized by the Internet Assigned Numbers Authority or Internet Engineering Task Force (IETF). However, it’s not reserved by either agency, so you can use it for your email transmission.
We recommend using 2525 only if you encounter problems with other ports. Port 2525 supports the Transport Layer Security (TLS) protocol to protect your message contents.
What about POP and IMAP?
Whenever you hear about SMTP, you’ll likely remember the two other main email protocols: Post Office Protocol (POP) and Internet Message Access Protocol (IMAP). These protocols, however, should not be mixed with SMTP as they serve another purpose.
POP and IMAP are used for retrieving a message from an SMTP server, while SMTP deals with sending it. Both protocols utilize their own ports, separate from SMTP.