There are four common SMTP ports: 25; 587; 465; 2525; Port 25 is commonly used for SMTP relay, but you should not use it for SMTP submission because most providers block it. If you want to configure your WordPress site or email client to use SMTP, you should start with port 587 as your first choice, as it’s the standard port for SMTP submission.
The RFC proposed a split of the traditional message submission and message relay concept. The RFC defined that message submission should occur over port 587 to ensure new policy and security requirements don't interfere with the traditional relay traffic over message relay port 25. It's easy to get started.
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25 is the default since E-mail was invented. It's also plaintext. 587 is more traditionally associated with TLS which is a more secure method of mail delivery but both client and server need to support it. List of TCP and UDP port numbers - Wikipedia, the free encyclopedia.
The concept of splitting the traditional message submission and message relay was proposed by the RFC. Thus port 587 was assigned to message submission to make sure that the new policy and security requirements don’t interfere with the traditional relay traffic over message relay port 25. You should use port 587 as a default SMTP port. Almost all mail servers support this port.
So there was a need for another port dedicated for “Submission”. RFC 2476 introduced port 587. It’s an exact replica of port 25 functionality. But requires authentication for mail submission (i.e. iPhone mail app to Gmail Server). In other words, port 25 is for “Public SMTP aka. Transfer”, port 587 is for “Private SMTP aka. Submission”
SMTP Port 587 . Whereas port 25 is the recommended port number for SMTP communications between mail servers (i.e., for relaying messages), port 587 is the one recommended for message submissions by mail clients to mail servers. To illustrate, In fact, this is stipulated in RFC 2476, which says that "Port 587 is reserved for email message submission...". Thus, port 587 is also known as the message submission port, while port 25 is also known as the message relay port.
Ports 25, 465, 587, or 2525 for SMTP have all been considered standard SMTP ports at some point, but only 587 or 2525 really should be considered for modern use. If you’re configuring your systems to use SparkPost as an SMTP relay, you should use port 587 as your standard SMTP port, with 2525 as an alternate in case port 587 is not available.
Relaying was entrusted to the good ol’ port 25 while all submissions were from now on to be directed via port 587. To this day, port 587 is the default SMTP port for most services and should be used whenever you need a STARTTLS port.