Email is like the first identity of every internet user since the journey of the World Wide Web began. Every day, approximately 392.5 billion emails (source: demandsage) are sent globally. Email is the foundation and still one of the most reliable communication tools to connect with anyone. Yes, sending an email is easier but sometimes you may wonder that how does an email actually transfer from your device to someone else, no matter where they are located? I think understanding SMTP and its ports is not only important for developers and email marketers, but also for anyone who is curious to know how this technology really works.
Today, I will try to explain what SMTP actually is, how SMTP ports work, and answer the most frequently asked questions from online forums and social networks. Hopefully, my explanation will give you a better understanding than reading a long technical description on Wikipedia or even ChatGPT.
What is SMTP?
Simple Mail Transfer Protocol, in short “SMTP”, is actually a set of communication rules that defines how emails are sent from one server to another. In simple words, SMTP is an outgoing courier that picks up your email and sends it to the right server by following the mail-sending rules. Sending mail is the main task of SMTP. It is responsible only for transferring emails from the sender’s server to the recipient’s mail server. SMTP is not responsible to store, reading or handle receiving messages.
For receiving emails, there are IMAP and POP3. IMAP stores emails on the server and synchronizes them across all devices. POP3 downloads emails to a specific (single) device so the user can access them locally.
If email is the post office, SMTP is the delivery truck that carries the mail to its destination, and that is its main task.
What are Ports related to SMTP?
If SMTP is the post office, then ports are the different doors inside it. An SMTP port is basically the network gateway your server uses to connect to an email server and send mail securely.
How SMTP Works (The Journey of a Message)
Usually an email goes through four invisible stage.
1. Submission: When you hit “Send”, your email first reaches the SMTP server. At this stage, your identity is verified through authentication, basic checks are performed, and the message is placed in a queue for delivery.
2. Routing & DNS Lookup: Next, the SMTP server figures out where the email should go. It checks the DNS records, especially MX (Mail Exchange) records, which act like digital postal addresses. These records tell the server which mail system is responsible for receiving the message.
3. Transfer: Now the real movement begins. The email travels from one mail server to another using SMTP. This is where SMTP ports come into play. The connection happens through one of the common gateways those are 25, 465, 587, or 2525. During this stage, the email also passes through security filters, spam checks, and reputation systems to ensure it’s safe and legitimate.
4. Delivery: Finally, the email reaches the recipient’s mail server. It is stored in the server, displayed in the inbox, and synced across all their devices so they can access it from anywhere.
You’ll often come across questions about SMTP setup, especially which port to use and which one is best. In this section, I’ll break down what each port actually does and which option is the most reliable for email marketing and everyday email delivery.
Port 25 – No longer for sending
Port 25 was introduced in 1982 and is now considered the “old school” SMTP port. In the early days of the internet, it was the primary channel for server-to-server email communication. Back then, the internet was small, and there was no need for authentication, encryption, or advanced spam protection.
However, as email usage grew, Port 25 became a popular target for spam relays, bot traffic, and unauthorized mail injection. Because of this, the modern internet changed significantly. Today, most ISPs block Port 25 for residential and even many commercial users.
Now, Port 25 is mainly used for server-to-server email relay, not for sending emails from applications, websites, or email marketing tools. It is generally not recommended for SMTP setup in modern systems.
Port 465 – The Premature Security Guard
Port 465 was originally introduced around 1990 for SMTPS (SMTP over SSL), using what is known as implicit SSL encryption. It became an important step in the evolution of secure email delivery and is still widely used today.
With Port 465, the connection is encrypted immediately from the very start and there is no negotiation phase. Security is established instantly as soon as the connection begins.
Later, the IETF standardized SMTP submission on Port 587 and marked Port 465 as deprecated for a period of time. However, the internet rarely forgets what still works in practice.
In modern usage, Port 465 has made a strong comeback. It is now commonly used with implicit TLS encryption (not legacy SSL). Although the terminology has changed, the underlying security is based on modern TLS standards. Many email providers still fully support it, rely on it, and continue to use it widely in production environments.
Port 587 – The Modern Highway
Port 587 is the modern SMTP standard and the default choice for most major email service providers today. Unlike older methods, Port 587 usually starts with a normal connection first and then upgrades it to a secure encrypted connection using STARTTLS (TLS encryption).
- The process is simple:
- Connect normally
- Authenticate the user
- Upgrade the connection securely with STARTTLS
- Send the email safely
Port 587 works reliably across ISPs, supports modern authentication systems, follows official RFC standards, and integrates smoothly with modern applications and APIs. That’s why it is often called the modern route of email delivery.
Port 2525 – The Trusted Back Alley
Port 2525 is not an official SMTP standard but in the real world, standards don’t always matter more than reliability. So networks block Port 25, restrict Port 587, internet with standard SMTP ports and that it way providers introduced Port 2525 as a fallback. It’s widely supported by Postmark, Mailgun, SendGrid etc.
Developers also prefer it because rarely blocked, performs like Port 587 and works globally across restrictive networks.
Port 25 Vs Port 465 Vs Port 587 Vs Port 2525
This is a very common confusion among email marketers and developers that what to choose and what the real difference actually is. In short, Port 587 is the modern standard and the best choice for most email setups. If your email provider specifically requires SSL-based SMTP or legacy support, Port 465 is perfectly fine. When Port 587 or even Port 25 is blocked by your hosting or ISP, Port 2525 becomes a reliable fallback option. And generally, Port 25 should be avoided for application or website email sending, as it is often restricted and not designed for modern authenticated email delivery.
FAQs
Which SMTP port is the most secure?
Port 587 should be the number #1 recommendation because it enforces authentication and uses STARTTLS to upgrade the connection to TLS encryption during the session lifecycle.
However, Port 465 is also secure, as it uses Implicit TLS, where encryption is established immediately upon connection.
Both are secure and supported by most email servers. The difference lies in how encryption is negotiated.
What is the actual difference between SSL and TLS in email?
SSL is the older encryption protocol, and TLS is its modern, more secure replacement.
Port 465 historically used SSL, but now it uses the Implicit TLS model.
Port 587 uses TLS via STARTTLS, which is an explicit upgrade method. TLS is more secure, more efficient in modern implementations, and is currently the industry standard.
Is port 25 safe for SMTP?
No. For email sending using an application or website, Port 25 is not safe. It has no built-in encryption and no authentication by default, and it is often blocked by ISPs and hosting providers.
However, it can be secured using STARTTLS encryption, but this depends on server support. It is usually used for mail server communication with other servers, such as MX to MX delivery and backend email routing.
What is the port 2525 used for?
Port 2525 is a fallback SMTP port for sending emails when Port 587 or Port 25 is blocked. It usually offers full TLS support and authentication. This is essentially an alternative solution. However, not all email service providers offer this port.
What is Port 2465 and Port 2587 in Amazon SES?
Port 2465 and Port 2587 are alternative SMTP ports used with Amazon SES when the standard ports (465 and 587) are blocked.
Port 2465 uses Implicit TLS, while Port 2587 uses STARTTLS, similar to Ports 465 and 587 respectively.
Is SMTP 587 SSL or TLS?
SMTP Port 587 uses TLS via STARTTLS, not SSL. The connection starts unencrypted and is then upgraded to secure TLS. SSL is outdated and not used on this port.
What is the Gmail SMTP port?
Gmail uses Port 587 with TLS encryption as the primary option. It also supports Port 465 with SSL/TLS (Implicit TLS) for secure email sending.
