Ports are the start and end points of any computer network. They exist to make sure that every process is identified, assigned to the right protocol, and take the correct route to fulfil their role.
If you’re running your email operation using an SMTP relay (rather than an API – more on that later), you will be using SMTP ports.
The port your SMTP is using can make a huge difference to your deliverability, so it’s worth understanding what they are, how they work, and which ones are best for you.
In this complete guide to SMTP ports, we’ll take you through everything you need to know about SMTP ports (and more besides!):
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What is an SMTP?
Before you can understand what an SMTP port is, you need to know what an SMTP is.
SMTP stands for ‘Simple Mail Transfer Protocol’. Put simply, it’s an application that sends, receives, and relays mail between senders and recipients.
When you send an email your SMTP server processes it, contacts the recipient’s SMTP server, and transfers the email. The recipient’s ISP then takes the email from the recipient’s SMTP and decides where in the inbox to place it.
What’s the difference between an SMTP and an ISP?
There are a lot of acronyms in the world of email, so it’s easy to get confused. SMTPs and ISPs work together closely, but they are not the same thing.
An ISP is an Internet Service Provider. ISPs are responsible for giving internet users a good experience. In the case of email, this means ensuring that their inboxes are kept free of things like spam and viruses.
The job of an SMTP is to transfer mail from server to server. The job of an ISP is to decide what happens to it after that.
To use a shipping analogy, if email is the ‘cargo’, SMTPs are the ships which take that cargo from island to island. ISPs are the on-land infrastructure and customs officials that help get that cargo into Inbox Island (or block it and send it back).
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What are SMTP ports?
There are hundreds of port types all over the internet. They help computers and applications understand the intended action, and funnel those actions through to the desired result.
For example, when you hop online and type in a web address, Port 80 kicks in. Port 80 directs the transfer of data between your computer and the web server you’re visiting.
SMTP ports have two closely related functions: to connect to a network, and to transfer data (in the form of emails) through that network. They allow the SMTP ‘ships’ to travel from place to place, and enable the ISPs to process the ‘cargo’.
These processes have two stages, which can occur separately from one another:
- Submission. As the name suggests, this involves submitting a message to an outgoing server. It’s like securing the cargo on the deck of the ship.
- Relay. Transfering the message between two servers. When the SMTP ship travels the sea from island to island, it is in ‘relay’.
As a general rule, you won’t need to worry about the ‘relay’ part. Your SMTP ship should take care of that without any need for you to configure it. However, it’s worth noting the difference because some older ports are more useful for relay than submission.
There are several SMTP ports. Which one should you use for sending emails? Let’s take a look:
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Which SMTP port is best to use?
There are four major SMTP ports. To decide which one is best for you, it’s worth taking a look at their history:
Port 25 – the old original
Port 25 is the oldest SMTP port around. It was established back in 1982, and was revolutionary for its time.
However, its time was 40 years ago, and a lot has changed since then.
Nowadays, Port 25 is a known haunt of spammers and other dodgy senders. Most residential ISPs and cloud hosts will block mail submitted by Port 25.
Port 25 is still useful for relay purposes, but ‘cargo’ submitted in Port 25 is unlikely to be accepted by the destination.
Port 465 – a deprecated and non-secure option
Port 465 was originally registered specifically for SMTPs. The purpose was to allow SMTPs to operate using the then-new SSL (Secure Sockets Layer) encryption protocol.
However, the port was only used in this manner for about a year, before being revoked in favor of encrypting SMTP communications with TLS (Transport Layer Security). With the addition of protocol command STARTTLS, Port 465 finally became irrelevant.
Some legacy systems may still need to use Port 465, but it’s too deprecated and insecure to be used if you can possibly avoid it.
Port 587 – today’s gold standard
Port 587 is the default port for email submission. If your default port is not set to 587, we strongly recommend changing it!
587 is securely encrypted, and widely trusted by ISPs everywhere. Using Port 587 will vastly improve your deliverability.
Port 2525 – the go-to alternative to 587
Unlike Port 587, Port 2525 is not endorsed by the IETF (Internet Engineering Task Force) or IANA (Internet Assigned Numbers Authority). However, unlike Ports 25 and 465, it is also not obsolete, deprecated, or insecure.
Port 2525 is a lot like Port 587, and can be used as an alternative if Port 587 is blocked.
Why should I care about SMTP ports?
It’s a fair question! Not everyone needs to concern themselves with which SMTP port they use.
SMTP ports are a relevant concern for you if you’re emailing via SMTP.
There are two main ways to send emails – via SMTP and via email API.
Briefly, the difference is:
- Sending via API. When sending with an API, you are sending through an application that communicates directly with other applications. APIs work through established app-data networks, so their data transfer protocols effectively cut out the ‘middleman’.
- Sending via SMTP. When sending via SMTP, your SMTP will create and send the data itself, following a set of predetermined rules. It’s more a set of rules through which communication can be achieved than it is an established communication interface.
If you’re sending via API, you don’t need to worry about SMTP ports. If, however, you’re sending via SMTP, it’s vital to configure your SMTP port in order for your emails to get delivered.
How can I check and configure my SMTP port?
Before you can optimize your SMTP sending, you need to know how to check and configure your SMTP port.
If you are subscribed to an email relay hosting service, the ‘support’ page should show you your SMTP server host name and port number.
However, if you have your own SMTP server, the configuration in ‘settings’ should show you the SMTP port number and host address.
To configure your SMTP port, you will need to go into the Outgoing Server menus. From there, select ‘Primary Server’ (or your SMTP’s equivalent), and ‘Server Port’ to change the port you’re using. You may need to enter your authentication password again before you can do this.
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