SMTP Ports: The web’s hidden highways — How emails travel? and why the wrong road can leave you lost?

Every email you send isn’t just a digital whisper — it’s a traveler, making its way across a hidden network of highways known as SMTP ports. Choose the right road, and your message arrives smoothly. Pick the wrong one? Your email might be blocked, delayed, or vanish into the void.

But here’s the catch — some highways are fast and open… but dangerous. Others are locked down, allowing only authorized travelers. Some have been shut down entirely. And only a few remain the gold standard for safe, reliable email delivery.

But here’s the problem — most people are driving blind. Many still rely on outdated, insecure, or even dead-end SMTP ports, unaware that cybercriminals lurk on those abandoned routes, waiting to hijack unprotected messages.

You wouldn’t send an important package through a postal route that shut down years ago. So why risk it with email? Let’s take a deep dive into how email actually moves through the internet, which SMTP ports are safe to use, and which ones can ruin your email deliverability.

SMTP and SMTP Ports

What is SMTP?

If SMTP ports are the highways, SMTP (Simple Mail Transfer Protocol) is the entire postal system behind email delivery. When you hit send, SMTP directs your message through an SMTP port. But getting through the port isn’t enough, your email must also pass authentication checkpoints. Fail them? Your message could be blocked, rejected, or sent to spam.

That’s why choosing the right SMTP port isn’t just about sending — it’s about ensuring delivery.

What is SMTPS?

SMTPS is to SMTP what HTTPS is to HTTP, a secure upgrade. Just like HTTPS encrypts web traffic to prevent unauthorized access, SMTPS (SMTP + TLS) encrypts email transmissions, to protect them from interception.

TLS (Transport Layer Security) acts like a digital signature, verifying where email traffic comes from before allowing it through and reinforcing authentication methods like SPF and DKIM.

The default port for secure email transmission is port 587. But before we dive into ports, let’s define what an SMTP port actually is.

What is an SMTP port?

As we agreed earlier, SMTP ports are the highways that emails travel on. They control how emails move across the internet.

Just like different roads are built for different vehicles, not all SMTP ports are created equal. Some are optimized for secure delivery, while others are outdated or even shut down altogether. Here’s a breakdown of the most important ones:

Port 25 — The original highway, now mostly blocked for outbound email.
Port 587 — The industry standard for email submission.
Port 465 — A discontinued road no longer widely supported.
Port 2525 — An alternative when others are unavailable.

Choosing the wrong port isn’t just a technical mistake — it can damage your reputation as a sender. Before we explore which ports to use (and which to avoid), let’s understand why there’s more than one SMTP port in the first place.

Why is there more than one port for SMTP?

Not all email highways serve the same purpose. Some roads are built for heavy-duty transport, while others are optimized for local traffic. SMTP works the same way — different ports exist because email follows different routes depending on where it’s going.

When you send an email, it doesn’t travel directly to the recipient. Instead, it goes into a multi stop journey:

Submission: The first stop of the road. Your email client submits the message to a mail server for processing. This typically happens over Port 587 — a secure, designated route for outbound mail.
Relay: The cross-country stretch. Once accepted, the message is transmitted between mail servers. The universal port for this is Port 25 — the backbone of server-to-server email transmission.
Receiving — The final delivery. But here’s the twist — SMTP isn’t used for receiving email. Instead, IMAP and POP take over, each with their own dedicated ports.

But why separate submission from relay? For two key reasons:

Preventing spam — ISPs block Port 25 for outbound mail to prevent spammers from sending emails directly from consumer networks. That’s why Port 587 is the go-to for email submission.
Security & authentication — Different ports enforce different rules. Submission ports (like 587) require authentication, while relay ports (like 25) are for server-to-server traffic.

By assigning different roads to different parts of the journey, SMTP keeps email traffic flowing smoothly, without getting lost, blocked, or hijacked.

Now that we know why multiple SMTP ports exist, let’s look at why choosing the right one matters.

Why your choice of SMTP port matters

Just like choosing the right road for your journey, selecting the right SMTP port determines whether your email reaches its destination, or gets stuck at a dead end.

When you connect to an SMTP service, you need to use a port that your provider actually supports. If you submit your email through a closed or unsupported port, your message won’t even leave the starting line.

The good news? Most mail servers keep multiple lanes open:

✅ Port 587 — The safest, and most reliable route.

🚫 Port 25 — The original but mostly blocked option.

⚠️ Port 2525 — A flexible alternative.

For nearly all cases, Port 587 is your best bet — secure, widely supported, and designed for email submission.

While Port 465 was once used for SMTP, most modern mail servers no longer support it.

Why?

Let’s break down each SMTP port and when (or if) you should use it.

Common SMTP ports and their use cases

Port 25: The original highway of email

Back in 1982, when email was still in its infancy, the Internet Engineering Task Force (IETF) laid down the rules of the road in Request for Comments (RFC) 821, assigning port 25 as the default port for sending email. It became the backbone of SMTP communication. It ensures reliable message transmission between mail servers.

At that time, security wasn’t the major concern as it is today, the focus was simply on delivering emails efficiently. But that same openness later made port 25 a prime target for spammers, who exploited its support for open relays to flood inboxes worldwide.

Fast forward to today, while port 25 remains the default port for SMTP relay (server to server transmission), it’s practically off-limits for email submission. Most ISPs and cloud providers block outbound traffic on port 25 to prevent spam abuse, making it impractical for everyday email sending.

Should you use port 25?

Unless you’re managing a mail server and handling server-to-server email relay, the answer is: No. Major email providers, including Gmail and Outlook, don’t support it for email submission, and modern email clients avoid it altogether.

Port 465: A locked-down highway you should no longer use

In the mid 90s, the Internet Assigned Numbers Authority (IANA) attempted to bring encryption into SMTP by assigning port 465 for SMTPS, a secure, SSL/TLS wrapped version of SMTP.

The goal was to establish a dedicated pathway for encrypted email transmission in response to rising security concerns as the internet evolved.

But here’s the twist: Port 465 was never officially recognized by the IETF. No formal RFC backed it up. Instead, the IETF pushed for STARTTLS, a more flexible way to encryption, leading to port 587 becoming the de facto standard for secure email submission.

Why does port 465 still exist?

Even though it was officially deprecated, some legacy systems, outdated applications, and older Microsoft products never fully moved on. Some consumer-facing email providers still list it in their setup guides, further adding to the confusion.

Should you use port 465?

No, unless you have no other choice. Modern hosting providers and email services generally don’t support it, and it’s not RFC-compliant. If encryption is your goal (which it should be), port 587 with STARTTLS is the correct path forward.

Port 587: The gold standard for secure email submission

By the late 1990s, port 25 had become a spammer’s playground. Open relays and botnet driven spam made it clear: email needed a better way to separate message submission (when a user sends an email) from message relay (when mail servers pass messages between each other).

Then came RFC 2476, a landmark proposal by R. Gellens and J. Klensin in December 1998, which officially designated port 587 as the new SMTP submission port. The goal? To separate legitimate email submission from server-to-server relay and introduce modern security standards to prevent abuse.

Why port 587?

Unlike its predecessors, port 587 was built with security in mind. It supports STARTTLS out of the box, allowing emails to be encrypted in transit, and shielding messages from prying eyes. More importantly, it’s RFC-compliant, which means it adheres to the internet’s best practices for safe and reliable email submission.

As a result, port 587 became the industry standard, widely adopted by email clients, hosting providers, and ESPs worldwide. Today, it’s the go-to port for sending emails. And yes — at MailTock, we recommend it.

Should you use port 587?

In almost every scenario? Yes. Whether you’re sending email through a web application, SMTP client, or ESP, port 587 is the correct choice. Major providers like Gmail, Outlook, and Apple Mail all default to it.

But — as with everything in email — there are exceptions. Some cloud providers, like Google Compute Engine, have historically blocked port 587 for outgoing mail. A handful of upstream networks may also restrict it, forcing users to look for alternatives.

Port 2525: The trusted backup when 587 won’t work

Sometimes, email just won’t cooperate. Port 587 is the industry standard, but what if your network blocks it? What if your cloud provider restricts it? That’s where port 2525 comes in — the unsung hero of SMTP submission, a reliable alternative that keeps your emails moving when the primary option is off the table.

It’s not officially recognized by IETF or IANA, yet it has quietly become the go-to fallback for modern email delivery. Most ESPs support it, and like port 587, it’s secure, supports TLS encryption, and ensures emails are transmitted safely.

Should you use port 2525?

If port 587 is blocked — whether by an upstream network, a restrictive ISP, or a cloud provider, port 2525 is your next best bet. Many hosting environments, including AWS, Google Cloud, and DigitalOcean, allow SMTP traffic over 2525, making it a practical choice when 587 isn’t available.

Headache-free SMTP sending with MailTock

Even with the right SMTP configuration, email failures can still occur. Misconfigured ports, authentication issues, and unexpected provider restrictions can silently disrupt critical email workflows — leading to missed transactional and marketing emails, customer frustration, and lost revenue. That’s where MailTock comes in.

Think of it as your real-time SMTP radar, continuously scanning for email failures and pinpointing issues before they escalate. Whether it’s a misconfigured port, a blocked connection, or an ISP silently rejecting your messages, MailTock ensures you’re never in the dark.

With instant failure notifications, actionable insights, and historical tracking, you don’t just react to email problems — you prevent them. And when an issue does arise? MailTock’s diagnostics provide clear, step-by-step solutions, helping you fix it fast. No more guessing. No more blind troubleshooting.

What’s Next? SMTP Ports, Security, and Common Issues Explained

Understanding SMTP ports is just the beginning. Security and troubleshooting are just as critical. In our next article, we’ll break down SMTP security, encryption, and common port-related issues — from authentication failures to ISP blocks — and how to fix them.

Stay tuned — because your emails deserve to be delivered, every time.