SMTP Port 465 vs. Port 587: What’s the Difference and Which One Should You Use?

When configuring email delivery for a website, marketing tool, or software application, one of the most overlooked — yet crucial — settings is the SMTP port. Without the correct configuration, emails may fail to send, end up in spam, or expose your communications to security risks.

Two of the most commonly used ports are 465 and 587. But which one is right for your setup? And why do both exist in the first place?

In this guide, we’ll explain:

What SMTP is and why ports matter
The history and function of ports 465 and 587
Key differences in security protocols
When to use one port over the other
How to configure your email settings correctly
What Mailpro recommends for the best results

What Is SMTP and Why do Ports Matter?

SMTP stands for Simple Mail Transfer Protocol. It’s the standard protocol used to send emails across the internet. Every time you send an email — whether it's through Gmail, Outlook, or a business tool like Mailpro — SMTP is involved in routing your message from one server to another.

SMTP needs a port to establish the connection. Think of a port as a door through which your email travels. Each port number serves a specific purpose and has varying levels of encryption and authentication.

Understanding Email Ports in Detail

Let’s take a closer look at the four common SMTP ports:

Port	Description	Encryption	Best Use
25	Default SMTP port, often blocked by ISPs	None / STARTTLS	Server-to-server
465	Legacy SMTPS	Implicit SSL	Obsolete / Legacy
587	Modern standard	STARTTLS	Client-to-server
2525	Alternative to 587	STARTTLS	Fallback if 587 blocked

The Origins of Port 465

Port 465 was initially introduced by Netscape in the 1990s to support SMTP over SSL (SMTPS). This meant the connection was encrypted as soon as it was opened — known as “implicit SSL.” However, this method was never standardized and was later deprecated by the IETF in favor of the STARTTLS protocol used on port 587.

Why It Was Deprecated

Implicit SSL connections do not allow for fallback or negotiation. If either the client or server doesn’t support SSL, the connection fails. As email systems evolved, flexibility and interoperability became essential — and that’s why port 465 fell out of favor.

The Rise of Port 587

Port 587 became the standard for email submission, using STARTTLS encryption. With STARTTLS, the connection starts in plain text, then upgrades to a secure connection after a negotiation. This allows for both flexibility and modern security.

Why STARTTLS Is Preferred

Improved compatibility with modern email systems
Allows fallback if encryption isn’t supported
Less likely to be blocked by firewalls or ISPs
Aligns with IETF standards like RFC 6409

Industry Adoption

All major email providers — Gmail, Outlook, Yahoo — and platforms like Mailpro recommend using port 587 with STARTTLS. It’s secure, flexible, and compliant with email delivery best practices.

Encryption Explained: Implicit SSL vs. STARTTLS

Port 465 – Implicit SSL

Connection is encrypted from the start
Fails if SSL not supported
Less flexible and more rigid

Port 587 – STARTTLS

Connection begins in plaintext
Upgrades to secure TLS connection
More secure against downgrade attacks
Supports fallback in complex systems

SMTP Port 465 vs. 587: Side-by-Side Comparison

Feature	Port 465	Port 587
Encryption	Implicit SSL	STARTTLS
Standardized	No	Yes (RFC 6409)
Security	Secure but outdated	Modern and flexible
Compatibility	Limited	High
Mailpro Support	Yes, not recommended	Yes, preferred
Fallback Options	None	Available

Common Misconceptions

“Port 465 is more secure.” While both ports offer encryption, port 587 with STARTTLS is preferred today for its flexibility, compatibility, and modern encryption practices.

“Port 25 is okay for sending.” Port 25 is reserved for server-to-server mail relays and is frequently blocked to reduce spam. Never use it for client submissions.

What If You Use the Wrong Port?

SMTP authentication may fail
TLS handshake errors
Higher chance of spam flags
Reduced deliverability

How to Configure SMTP with Port 587 (Example: Mailpro)

1. Log into your Mailpro account

2. Go to Settings > SMTP Parameters

3. Use the following credentials:

SMTP Server: smtp.mailpro.com
Port: 587
Encryption: STARTTLS
Username: your Mailpro email
Password: your Mailpro password

4. Enable authentication and save your changes

How Mailpro Ensures SMTP Reliability

With Mailpro’s SMTP relay, you benefit from:

Full support for STARTTLS encryption
Spam testing and bounce tracking
Dedicated IP pools for reputation control
Real-time analytics on email delivery

Troubleshooting SMTP Port Issues

Email not sending? Check the port and encryption method
Authentication errors? Confirm username and password
Firewall blocking? Use port 2525 as a fallback

Final Recommendation

Port 587 is the modern, secure, and officially supported port for email submission. It supports encryption, is rarely blocked, and ensures high deliverability. Port 465 is supported only for backward compatibility and should be avoided unless strictly required.

Frequently Asked Questions

Can I use port 465 with Mailpro?

Yes, but it’s not recommended. Mailpro advises using port 587 for better security and compatibility.

What if port 587 is blocked?

You can use port 2525, which is also supported by Mailpro and offers STARTTLS encryption.

Does Mailpro require SMTP authentication?

Yes. You must log in with your Mailpro email and password to send messages via SMTP.

Conclusion

Understanding the difference between SMTP port 465 and 587 is critical for ensuring email deliverability and compliance with modern standards. Port 587 with STARTTLS provides a secure, flexible, and reliable way to send emails, and it’s the configuration recommended by Mailpro and all major providers. Stick with 587 to ensure your messages reach the inbox — not the spam folder.