If you’re configuring email services on a VPS and find that outgoing messages aren’t being delivered, the problem may be tied to Port 25. This port, once the standard for email transmission, is now widely blocked by many hosting providers and ISPs due to its association with spam and abuse. In this article, we’ll explain what Port 25 is, why it’s blocked, and what alternatives you should use for secure and reliable email sending.

What Is Port 25?

In our previous article about Port 25, we explored the fundamentals of SMTP Port 25, its purpose in email transmission, and its historical significance in server-to-server communication.

To further expand on that foundation, this article will explain why Port 25 is now widely blocked, the risks associated with using it, and which secure alternatives like Port 587 you should use for sending emails from your VPS.

Port 25 is the default network port used for SMTP (Simple Mail Transfer Protocol), which is the protocol responsible for sending emails between mail servers.

  • Port Number: 25
  • Protocol: SMTP
  • Purpose: Server-to-server email delivery (MTA-to-MTA communication)
  • Security: Typically unencrypted (unless wrapped in SSL/TLS manually)

In the early days of email, Port 25 was used not just for server-to-server communication but also by email clients to submit outbound messages. However, this changed over time due to growing security concerns.

Why Is Port 25 Blocked?

Port 25 has become one of the most abused ports on the internet. Spammers, malware, and bots frequently exploit it to send large volumes of unsolicited email. For this reason, most major cloud VPS providers, including AWS, Google Cloud, Azure, and even some ISPs, block outbound connections on Port 25 by default.

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Reasons why Port 25 is blocked:

  1. It does not require authentication: Unlike modern email submission ports such as 587 or 465, Port 25 doesn’t require authentication by default.
  2. Traffic over Port 25 is usually unencrypted
  3. Easily exploited for bulk spam sending: Spammers looking to send bulk email anonymously.
  4. Makes it difficult to trace malicious users
  5. Violates anti-spam and abuse policies of cloud providers

This opens the door to man-in-the-middle attacks, IP blacklisting, and unauthorized mail relays, making Port 25 a major liability.

Google Cloud’s official documentation also states Port 25 is blocked, recommending alternative ports like 587 and 465.

What to Use Instead of Port 25

If you’re running a mail server or sending email via your VPS, you should use the recommended alternatives:

Port 587 (SMTP with STARTTLS)

Port 587 is the official port for email submission. It requires authentication and uses STARTTLS to encrypt the communication between your email client and the SMTP server, meaning email clients (like Outlook or Thunderbird) or applications sending email must use this port.

  • Protocol: SMTP with STARTTLS
  • Authentication Required: Yes
  • Encryption: STARTTLS
  • Purpose: Outbound email from client to mail server (MSA-to-MTA)

Read our detailed guide: What is Port 587 and How to Open It

Port 465 (SMTP with Implicit SSL)

Port 465 is sometimes used for SMTP over SSL, though it’s considered deprecated by IETF. However, some providers still support it for secure email submission.

  • Encryption: Implicit SSL
  • Authentication: Yes
  • Supported By: Gmail, Yahoo, Zoho Mail, and others

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Can You Request to Unblock Port 25?

Some VPS providers offer the option to unblock Port 25 upon request, but you must provide a valid reason and explain your intended use (e.g., for transactional email or business communication). Even then, approval is not guaranteed.

At vpswindows.com, we prioritize network integrity and IP reputation. Therefore, we recommend clients to use Port 587 with SMTP authentication for any email-sending activities.

Security Risks of Using Port 25

Even if it’s not blocked, using Port 25 for email transmission poses serious risks:

  • Lack of encryption exposes credentials and data
  • Vulnerable to packet sniffing and MITM (man-in-the-middle) attacks
  • High risk of blacklisting due to spam-like behavior
  • Weak compliance with security standards like DMARC, SPF, and DKIM

That’s why compliance-focused organizations and email service providers always recommend using authenticated and encrypted ports like 587.

Email Configuration Best Practices

To ensure reliable and secure email delivery from your VPS:

  • Use SMTP authentication
  • Configure SPF, DKIM, and DMARC records
  • Use Port 587 with TLS
  • Avoid self-hosting mail servers unless necessary
  • Use reputable third-party SMTP relay services if needed (e.g., SendGrid, Mailgun, Amazon SES)

Conclusion

Port 25 and why it’s blocked is no longer just a technical issue—it’s a critical security policy embraced by nearly every modern cloud provider. Port 25 may still play a role in mail server communication, but for most users and especially for sending mail from a VPS, it’s a relic of the past. Its widespread blocking is a result of years of abuse, and rightly so. For a secure and modern email setup, you should use authenticated ports like 587 and configure your VPS accordingly.

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