SMTP ports play an important role in sending SMTP (Simple Mail Transfer Protocol). That is why it is important to choose the right port when configuring an e-mail server. Incorrect configuration can cause errors when sending and receiving e-mail.

This article will describe what SMTP ports are used for and analyze four of them. We will also explain their functions and features to help you choose the right port for your SMTP server.

Table of Contents view
What exactly are SMTP ports used for?
SMTP is a mail-sending protocol that allows users to send e-mail over the Internet, and SMTP ports act as communication endpoints that handle information transfers from one server to another.

To begin using an SMTP server, you must set up your e-mail account. This can be done by entering an IP address and port number to properly route your emails.

There are three stages that emails must go through before they reach the recipient:

  • Sending e-mail : sending e-mail to an outgoing mail server. This happens via SMTP, so you will have to decide which port to use.
  • SMTP forwarding : forwards messages between two mail servers. Usually, users do not have to configure a port because it uses a default one.
  • E-mail reception : e-mails arrive at the recipient’s server. This process is handled by Internet Message Access Protocol (IMAP)/Post Office Protocol 3 (POP3).

Which SMTP port to use to connect to the mail server?
Each SMTP port supports at least one level of encryption that determines the security of the e-mail transmission process. When choosing a port, be sure to choose the one that best suits your needs.

Four levels of encryption are available:

  • Secure Sockets Layer (SSL). SSL encryption is an outdated technology that protects e-mail transmissions between two SMTP servers.
  • Transport Layer Security (TLS). TLS is the successor to SSL that encrypts Internet traffic using modern, secure systems.
  • Opportunistic TLS (STARTTLS). STARTTLS is a protocol command used to upgrade an insecure connection to a secure one using SSL/TLS.
  • Null (NONE). No encryption provided.

Here is a table of each SMTP port with its type of encryption:

Use of unencrypted SMTP port 25.
Initially, the Internet Engineering Task Force (IETF) assigned SMTP port 25 as the standard transmission channel for e-mail.

Although the Internet Assigned Numbers Authority (IANA) still recognizes it as the default SMTP port, it is not recommended to use it as a channel for sending mail. In fact, port 25 has been used to spread spam and malware.

In addition, port 25 does not encrypt messages, transmitting information in plain text. Since threat actors can intercept unencrypted data, this raises many security concerns.

As a result, many Internet service providers (ISPs) and web hosting have decided to block any traffic on port 25. This is done to reduce unwanted e-mail sent through it.

That said, port 25 is a good channel for testing the connection or troubleshooting e-mail configuration problems. For example, if the server does not respond using port 25 when configuring your Web site’s contact form, this indicates a problem with the non-SMTP server.

Sending mail using SMTP port 465
Port 465 serves as a better alternative to 25 in case port 587 does not work.

IANA introduced SMTP port 465 as an upgrade to port 25. It facilitates the secure transmission of e-mail using Simple Mail Transfer Protocol over SSL (SMTPS). However, the IETF never officially approved it as an official SMTP port.

Sending mail via port 465 is protected using implicit TLS, which means that the initial connection is encrypted using an SSL or TLS certificate.

Currently, SMTP port 465 is obsolete and no longer considered an accepted standard for sending mail. We recommend using it only if your hosting provider requires the protocol.

Sending messages via SMTP port 587
SMTP port 587 was introduced as the successor to channel 465. Both IANA and IETF recognize it as the current default SMTP port for sending e-mail.

SMTP port 587 supports STARTTLS, which means it also works with SSL and TLS encryption. It allows messages to be sent securely and provides a level of protection from spam and malware.

STARTTLS is a protocol command issued between an e-mail program and a server. It encrypts information transmitted using TLS connections. In addition, STARTTLS offers SMTP authentication, which requires the SMTP client to authenticate with the mail server to gain access to the system.

SMTP port 587 is currently the best option and we recommend that you use it for sending e-mail as it provides a more robust security mechanism than the other ports.

Try the non-traditional SMTP port 2525
Port 2525 is a modern protocol used for connecting to SMTP servers. It is considered a secure option because it supports TLS encryption for e-mail transmission.

Nowadays, it is a commonly used port supported by many ESPs, ISPs, and cloud hosting services. In some cases, port 2525 is used to configure virtual private servers (VPS).

Port 2525 serves primarily as an alternative if you experience connectivity problems when sending SMTP. If you use port 587 but the connection is blocked or rejected, try switching to port 2525 instead.

However, port 2525 is a nonstandard channel, so it is not widely monitored by IANA and IETF-it is not recognized as an official SMTP method. In addition, some hosting providers also do not support it.

Conclusion
During the e-mail transfer process, different ports are used to send e-mail to one outgoing mail server and forward messages from one SMTP server to another.

SMTP is a protocol that allows you to send e-mail over the Internet. SMTP ports help route messages to the right server and act as communication endpoints between mail servers.

Choosing the right port is important in order to send e-mail. Here is a summary of the four SMTP ports:

  • Port 25 : has no security mechanism and therefore is not recommended for sending e-mail. It is currently used as an SMTP forwarding port.
  • Port 465 : This port was designed for SMTPS since it used SSL encryption. However, IANA stopped supporting the protocol and replaced it with STARTTLS encryption.
  • Port 587 : The successor to port 465 and currently the default transmission channel for SMTP. Port 587 provides secure e-mail transmission using STARTTLS encryption.
  • Port 2525 : An alternate port to be used in case of connection problems with port 587. However, it is not recognized by IANA and IETF as an official SMTP port.

We hope this article has helped you choose the right SMTP port.