SMTP Ports Explained: 25, 465, 587 (and 2525)

What is an SMTP port?

An SMTP port is the numbered channel your server uses to send email. SMTP stands for Simple Mail Transfer Protocol, the standard for moving email across the Internet. The port number tells the server which “door” to use for that traffic.

Think of an IP address as a building and the port as a specific door in it. Email submission uses one door, web traffic uses another. Using the right port matters because servers listen for email only on specific channels.

Each SMTP port also signals how the connection is secured. Some ports expect encryption to start on the first byte. Others start in plain text and upgrade. We’ll cover that difference below.

SMTP ports use TCP, not UDP. If you’re wondering whether SMTP ports are TCP or UDP, the answer is always TCP. This is because email delivery requires reliable, ordered transmission.

How do SMTP ports relate to sending vs. receiving email?

SMTP handles sending, not receiving. When your app hands an email to a mail server, that’s called submission. When one mail server passes the message to another, that’s called relay.

Receiving email uses different protocols and ports. IMAP uses port 993, and POP3 uses port 995. So if you’re pulling mail into an inbox, you’re not using an SMTP port at all.

The focus here is sending. That means ports 587, 465, 25, and 2525.

Which SMTP port should you use?

The most common SMTP ports are 587, 465, 25, and 2525, each with a specific role in sending email.

Use port 587 for almost everything. It’s the standard port for authenticated email submission, and it uses STARTTLS to encrypt the connection. Nearly every email client, app, and sending service supports it.

If your setup only supports SSL or the server blocks requests from port 587, use SMTP port number 465 instead. Save port 2525 as a fallback when both are unavailable. Avoid port 25 for submission, since providers block it heavily.

Here’s a quick comparison:

What is port 587, and why is it the default?

Port 587 is the standard port for email submission, and it’s the one you should reach for first. SendLayer SMTP server uses it as well.

It was defined in RFC 6409 specifically for mail clients submitting messages to a server. That’s exactly what your app or plugin does.

Today, the SMTP default port number for email submission is 587, replacing the older use of port 25 for most applications.

The connection starts in plain text and upgrades to TLS before credentials are transmitted. Your username and password stay protected.

It also expects authentication. You have to log in with valid credentials, like an SMTP password for SendLayer or an app password for providers like Gmail, to send. That requirement is a big reason spam is harder to push through port 587.

What is port 465, and is it deprecated?

Port 465 is a submission port that uses implicit TLS, and it’s still perfectly usable today. The connection is encrypted from the very first byte, with no plain-text handshake. It’s sometimes called SMTPS.

Its history is messy. Port 465 was first assigned to SMTPS, then deprecated in favor of 587, and later formally reinstated for submission in RFC 8314.

Use port 465 when your client or host only supports SSL connections. Many providers, including SendLayer, support both 587 and 465. Pick whichever your software handles cleanly.

What is port 25 used for?

25 is one of the common SMTP port numbers used for relay between mail servers, not for submitting email from your app. When one mail server delivers a message to another server, it uses port 25. That’s its original and proper job.

The problem is spam. Attackers abused port 25 for years to blast unauthenticated mail. In response, most ISPs and cloud hosts now block outbound port 25 by default.

Don’t use port 25 to send from an application or client. Your connection will likely time out or get refused. Reserve it for the mail-transfer-agent traffic it was built for.

When comparing SMTP port 25 vs 587, the key difference is use case: port 25 is for server-to-server relay, while port 587 is designed for authenticated email submission from apps and clients.

What’s the difference between STARTTLS and implicit TLS?

The difference is when encryption starts. STARTTLS begins in plain text, then upgrades the connection to TLS. Implicit TLS encrypts the connection from the first byte.

Both end up encrypted, so both are secure when set up correctly. Port 587 uses STARTTLS while port 465 uses implicit TLS.

Here’s the short comparison:

How do you find and change your SMTP port?

You can change the SMTP port in your email settings or SMTP settings, depending on your plugin, app, or email client.

The exact location varies, but you’re always looking for a field labeled “Port” or “SMTP Port” next to the encryption setting. Change both together, since the port and encryption type must match.

WordPress

In WordPress, SMTP plugins expose these options directly in your SMTP settings, including the port, host, and encryption type.

This is particularly helpful if you’re using the Other SMTP mailer with SMTP plugins like Easy WP SMTP.

If you switch encryption to SSL, the port field updates to 465. Save your changes, then send a test email from the Email Test tab. A successful test confirms that the port and encryption are correct.

Common email clients and apps

In email clients and custom code, you set the port in your outgoing SMTP server settings. Look for the host, port, and encryption fields together. A typical SendLayer configuration looks like this:

Host:        smtp.sendlayer.net
Port:        587
Encryption:  STARTTLS (TLS)
Username:    $SENDLAYER_USER
Password:    $SENDLAYER_PASS

Keep your credentials in environment variables, not hardcoded in files. That prevents your username and password from leaking into version control or client-side code.

What should you do if port 25 or 587 is blocked?

If a port is blocked, switch to another submission port before anything else. Try 587 first, then 465, then 2525. If your WordPress site is still not sending emails, a blocked port is usually why.

First, confirm the port is actually blocked. Run this command to test whether the port is open:

# Test if the SMTP port accepts a TLS connection
openssl s_client -connect smtp.sendlayer.net:587 -starttls smtp

If the connection hangs or is refused, the port is blocked on your network. Switch your app or plugin to port 465 or 2525 and test again. If every port fails, your host is likely blocking outbound mail, and you should use an authenticated sending service over its supported ports.

Which SMTP ports does SendLayer use?

SendLayer supports ports 587 and 465 for SMTP sending. Port 587 with TLS is the recommended setup, and 465 with SSL works when you need implicit TLS.

SendLayer is a transactional email service built for developers and website owners. It handles the SMTP connection, authentication, and domain setup so your email reaches the inbox. You can connect over SMTP or use the API.

For most sites, connect over SMTP on port 587. If you’re sending at a higher volume or want more control, the API is the better fit. See our developer documentation for more information on using the email API.

FAQs — Understanding SMTP port

Below, we’ve answered some of the top questions about SMTP ports.

David Ozokoye

David is a technical writer at SendLayer. He tests and documents new features and updates to SendLayer's email services. Away from the computer, he enjoys playing video games and roller skating.

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