Why port 25 is blocked?

Port 25 is one of the most common ports that get blocked by internet service providers and organizations. This port is associated with SMTP or Simple Mail Transfer Protocol, which is used for sending outgoing emails.

Why Port 25 is Blocked?

There are a few key reasons why port 25 gets blocked:

Security and Spam Prevention

One of the main reasons port 25 gets blocked is for security and spam prevention purposes. Since this port deals with email sending, leaving it open can make networks vulnerable to:

  • Email viruses and malware – An open port 25 allows cybercriminals to gain access and send malicious emails or viruses.
  • Spam attacks – Spammers can exploit the open SMTP port to send unsolicited bulk emails.
  • Email spoofing – Hackers can disguise their sender address and send fake or phishing emails.

Blocking port 25 closes this vulnerable entry point and prevents the network from getting exploited for malicious emails and spam.

Unauthorized Use of Mail Servers

With an open port 25, anyone can use the mail server to send emails even without authorization. This allows spammers to exploit the mail server and bombard recipients with unsolicited emails. By blocking this port, networks can control who gets to use the SMTP server for outbound emails.

Comply with ISP Restrictions

Many internet service providers block port 25 on consumer networks to comply with regulations and prevent spammers from exploiting home internet connections. Standard consumer internet plans do not allow running mail servers. So ISPs block port 25 to restrict this.

Prevent SMTP Open Relaying

If port 25 is left open on a poorly configured mail server, spammers can exploit it for “open relaying”. This means external hosts can route emails through the server to send spam. By blocking port 25, networks prevent misconfigured mail servers from becoming an open relay for spam delivery.

So in summary, the need for security, preventing unauthorized use, complying with ISP policies, and avoiding open relaying issues prompt many organizations and ISPs to block port 25 on their networks.

How Port 25 Blocking Affects Legitimate Users

While blocking port 25 has its security advantages, it can cause issues for legitimate users trying to send emails:

  • Outbound email failure – Any application or website trying to send outbound emails directly using port 25 will fail. This includes contact forms on websites, software updates that email reports, application notifications sent over email, etc.
  • On-premise mail servers affected – Organizations running their own SMTP mail servers for outbound emails will find these servers blocked and unable to deliver messages over port 25. They will have to rely on ISP provided servers or implement alternative configurations.
  • Inconvenient authentication requirements – Even when an ISP allows port 25 traffic after authentication, users may find it inconvenient to provide these authentication credentials in applications that try to send emails.
  • Difficult troubleshooting and support calls – A blocked port 25 often results in support calls from confused end-users about why their email is failing. Troubleshooting such email sending issues often requires checking if port 25 is allowed or blocked on the network.

So while better for security, a blocked port 25 creates its share of issues for legitimate email usage which need to be addressed differently.

Solutions and Alternatives to a Blocked Port 25

Here are common solutions and workarounds when faced with a blocked port 25 either on ISP connections or organizational networks:

Use Port 587 instead of 25

Port 587 is the standard submission port for SMTP mail. Most ISPs leave this port open even while blocking port 25. Unlike 25, port 587 adds transport layer security and client authentication to make the email sending process more secure.

So applications that need to send emails can be reconfigured to connect through port 587 instead of port 25. Using submission port 587 with TLS and authentication allows legitimate email usage without opening security issues.

Use ISP Provided SMTP Relay

ISPs often provide an alternate SMTP relay server for their customers to send outbound emails when they block port 25. Customers can route their application emails through the ISP SMTP relay rather than running their own mail server.

For example, residential ISP plans usually include an SMTP relay server for sending account emails while blocking home usage of port 25. Using this ISP system is an easy alternative.

Configure Smart Host SMTP Relaying

Organizations running an on-premise mail server can configure an external “Smart Host” SMTP relay server to send outbound emails instead of delivering directly from their servers to destination servers over port 25.

The on-premise mail servers are configured to route all outbound external messages to the Smart Host server relay instead of connecting directly to external recipients. This relay server will deliver the messages to recipients over port 25 on behalf of the on-premise mail server.

Maintain Access List for Port 25

Network admins can maintain allowlists of static IP addresses that are allowed to access port 25 through the firewall while blocking the port in general.

For example, on-premise mail servers can be added to an IP allowlist to send emails over port 25 to external domains even if the port is blocked for other systems. This restricts exposure while still giving necessary access to applications that require port 25 availability.

So in conclusion, alternatives like port 587, ISP provided SMTP relays, smart host relaying, and access lists can address legitimate email requirements when faced with blocking of the vulnerable port 25.

Key Takeaway

  • Port 25 blocking is commonly done for security against email viruses, spamming, spoofing and unauthorized mail server access. But it affects legitimate email delivery too.
  • Alternatives like submission port 587, using ISP SMTP relays, configuring smart host relaying and allowlists help restore email functionality when port 25 is blocked.
  • Legitimate applications also need reconfiguration sometimes to route emails through submission port 587 or ISP provided SMTP relays instead of trying to deliver directly over blocked port 25.


Port 25 is routinely blocked across networks and ISPs to prevent security issues from malware, spamming and unauthorized usage. But this affects applications and services trying to send legitimate outbound emails leading to delivery failures.

Using alternatives like submission port 587 with authentication, ISP provided SMTP relays, smart host relay configurations and access lists can restore email functionality. Some reconfiguration is also needed for applications directed to use port 25 for emails instead of blocked port 25.

With the proper alternatives and configurations, critical email delivery can be maintained while keeping networks secured against vulnerabilities of an open port 25.

Meta Description: Understand why port 25 gets commonly blocked by ISPs and organizations for security and spam prevention. Also learn workarounds that can be implemented to allow legitimate emails.

Frequently Asked Questions

Q: What is port 25 used for?
A: Port 25 is used for SMTP or Simple Mail Transfer Protocol. It allows applications and mail servers to send outgoing emails via this port.

Q: Is blocking port 25 illegal?
A: No, blocking port 25 is not illegal. ISPs commonly block it to comply with regulations and prevent spam abuse. Companies block it for security against email malware and phishing.

Q: Can port 25 be opened?
A: Port 25 can be opened by requesting your ISP. Companies can also open it for their mail servers and whitelist internal IP addresses. But keeping port 25 closed is recommended by security experts.

Q: What happens when port 25 is blocked?
A: Outgoing emails will fail when port 25 is blocked. On-premise mail servers, contact forms, email alerts from applications will not be able to deliver messages leading to failures.

Q: Why would an ISP block port 25?
A: ISPs block port 25 to comply with regulations preventing consumer internet connections from running mail servers. It also saves support costs dealing with spam complaints. And it improves overall security against spam and phishing issues.

Q: How do I know if port 25 is blocked?
A: Try sending an email directly via Telnet on port 25 using the mail server. If the connection fails or times out, port 25 is likely blocked by your ISP or firewall policies.

Q: What port should I use instead of 25 when blocked?
A: Use SMTP submission port 587 instead of port 25 for outgoing emails when 25 is blocked. Port 587 adds security using TLS encryption and authentication to send emails.

Q: Can I use Gmail SMTP server when port 25 blocked?
A: Yes, Gmail SMTP server can be used as an SMTP relay or smarthost when your port 25 is blocked. Route your on-premise mail server traffic through Gmail for delivery.

Q: What is an SMTP relay?
A: An SMTP relay is an intermediary mail server that receives messages from a blocked mail server and forwards them externally to recipients over port 25 on its behalf. ISPs provide SMTP relays for customers to route emails.

Q: What is SMTP port 587?
A: Port 587 is known as the SMTP submission port or MSAs. It has added TLS encryption, authentication and spam filtering capabilities compared to port 25. It is the recommended alternative for sending outbound emails.

Q: How do I configure SMTP relay?
A: You can configure SMTP relay or smart host settings in your email client, mail server, or application interface. Add the relay server’s SMTP details including server, port 587, TLS and authentication credentials.

Q: Can I have port 25 open for one IP address?
A: Yes, network security admins can configure firewall policies to restrict Port 25 to one trusted IP, usually assigned for the mail server while blocking it for all other systems and IPs.

Q: What are best practices for port 25?
A: Best practices include closing port 25 except select allowlist IPs, using submission port 587 for outbound emails, employing SMTP relay, enhancing server security, blocking unused protocols, enabling authentication, etc.

Q: Is port 25 TCP or UDP?
A: Port 25 uses TCP or Transmission Control Protocol for SMTP email delivery. This allows reliable and error-checked delivery unlike UDP which has no delivery guarantees.

Q: What port replaces port 25?
A: Port 587 replaces port 25 in most situations when 25 is blocked. Port 587 is for SMTP Submission traffic and supports encryption + authentication unlike port 25.

Q: Can my IP be blacklisted if port 25 is open?
A: Yes, spammers can exploit open port 25 to send spam which can get your public IP address blacklisted by anti-spam groups if it relays unsafe, unauthenticated traffic.

Q: Is port 25 blocked on residential internet?
A: Most residential ISP plans block port 25 to comply with regulations preventing home connections from running mail servers. Consumers will use ISP’s outgoing SMTP relay instead.

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