What Port Does SMTP Use

What is a Port?

If you are new to email and want to learn more about different port options and what port SMTP uses, then we need to start with the basics and learn what a port is.  It all starts with computers talking to one another…wait what? It’s true, computers love to talk…a lot…ok, maybe not as much as Jerry in the cubicle next door, but they communicate more than you may know.  As a matter of fact, almost anything you do on your computer or phone requires that device to communicate with another. For example, something as simple as sending an email requires a ton of inter-device communication.

The process of communication between devices with a predetermined set of rules and procedures is called a protocol. These unique protocols communicate through what we call ports.  Think of kids talking through two soup cans connected by string. The cans represent the devices and the string represents the port – one device sending a message to another through a medium of communication.

What Port does SMTP use?

SMTP or Simple Mail Transfer Protocol is an internet protocol that is responsible for the delivery of email.  This protocol uses SMTP servers that communicate with one another to send an email from a sender to a recipient.  Typically this communication of SMTP servers will happen over port 25. Port 25 is the standard channel used to transmit email communication. Unfortunately, because port 25 is the default channel for email delivery, it is often abused by people with malicious intent.  Because of this, many mailbox providers like MSN, Verizon, and others have decided to block port 25 all together.

Why is Port 25 Blocked?

As mentioned, port 25 is the default communication channel used to send email.  Because it is standardized, it has a number of vulnerabilities that have caused major internet service providers to block its usage.  Lets say a hacker gets in to someones computer and takes over its functionality. That bad actor can easily use port 25 to blast out thousands of malicious spam or phishing emails through the victims device.  This vulnerability has been exploited enough for internet service providers to take action against it. Since there isn’t much they will do to improve the security of port 25 itself, they block it, leaving customers to seek alternative options.  So what can a bulk email sender do if port 25 is blocked?

What to do if Port 25 is Blocked?

Operating your own email system and server can be a hassle, many bulk email senders will enlist the help of an email service provider like SocketLabs.  An email service provider will have the necessary infrastructure and email know-how to help senders successfully deliver their marketing and transactional emails.  Part of this “know-how” means that they will have different port options for sending email in the event that a customer is blocked through port 25. Different email service providers may use different ports to send and receive email,  let’s discuss some of these alternative ports used by SMTP.

Popular SMTP Ports

Port 25

At SocketLabs, we will typically send and receive email through port 25.  Port 25 is still widely used for SMTP relaying, meaning it’s the best port to use for email that is being transmitted from server to server. This allows us to better manage the email traffic we support, and help our customers achieve maximum deliverability. Port 25 also has an option to be encrypted through an SMTP extension called STARTTLS for added security.

Port 2525

If you can’t connect over the standard port 25, you can use the alternative port 2525. Most applications support custom port configurations, so this is often a simple solution to the aforementioned problem.  Like port 25, 2525 also has the potential to be encrypted through the same SMTP extension.

Port 587

Port 587 is the standard mail submission port, meaning it is specialized in the delivery of email from a mail client to a mail server, compared to server to server communication in port 25.  Port 587 can also be encrypted similar to the previously mentioned ports

Port 465

Port 465 is similar to port 587 and can be used as an alternative.  465 is encrypted as well and is secure. Port 465 uses transport layer security or TLS to encrypt communications with an SMTP server. TLS security was a solution to encrypting SMTP traffic before the STARTTLS extension was added to the SMTP protocol. While port 465 has been deprecated in favor of the STARTTLS extension, it is still widely supported and available.

If you’re wondering how to change your SMTP port, an email service provider like SocketLabs will provide an easy way to manually switch ports from within your unique application.  If you aren’t working with an email service provider, each internet service provider will have their own way to switch the port you are sending on.

Visit our SMTP homepage to learn more about our SMTP server.