How to Check If Port 25 Is Open?

Port 25 is the backbone of how email gets sent across the internet using the Simple Mail Transfer Protocol (SMTP). When you’re managing a mail server or simply trying to figure out why your emails aren’t going out, checking if port 25 is open can be the difference between seamless communication and frustrating silence.

When port 25 is blocked or closed, say by your firewall, ISP, or server configuration, it prevents email-sending applications from connecting to external SMTP servers. This results in failed deliveries, timeouts, or even bounced messages.

In this article, we’ll walk you through how to check if port 25 is open across different systems: Windows (including Windows 10), Linux, and macOS. We’ll also cover alternative SMTP ports, such as 587, and share multiple tools and methods, including options for using Telnet, Netcat, or online checkers.

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RELATED ARTICLE: Is Hardware Technology Important for Cybersecurity?

What Is Port 25 and Why It Matters?

Port 25 is the original default port used by the Simple Mail Transfer Protocol (SMTP), the standard protocol for sending email across networks. When an email is sent, especially from server to server, it usually travels through port 25. This makes it critical for any service that handles outbound mail.

But here’s the catch: because port 25 has been widely abused for spamming and email spoofing, many Internet Service Providers (ISPs) and cloud hosting platforms now block it by default, especially on residential or consumer plans. That means even if your local server is set up correctly, emails might still fail to send because the network itself is blocking the port.

It’s also important to understand the alternatives:

• Port 587 is commonly used for sending mail with authentication (SMTP submission).
• Port 465 was an early attempt at secure SMTP over SSL, but it’s now deprecated in favor of 587 with STARTTLS.

In essence, knowing how to check if port 25 is open, SMTP is crucial for diagnosing email delivery issues, especially if you’re running a mail server, a cloud-based app, or a marketing tool.

Common Signs That Port 25 Might Be Blocked

Not sure if port 25 is blocked? Your system might already be dropping hints.

Here are the most common signs:

1. Email Sending Fails Repeatedly

You’ve configured your SMTP settings, but emails still aren’t going out. If your application or server logs show persistent errors like “Connection timed out” or “Could not connect to SMTP host,” port 25 may be blocked.

2. Bounce-back Errors from Mail Servers

You might receive automatic replies with status codes such as 421, 450, or 554, indicating that the server couldn’t establish a connection to the recipient’s SMTP server. These often trace back to blocked outgoing connections on port 25.

3. Telnet Fails to Connect

If you’re using Telnet to test SMTP (e.g., telnet smtp.domain.com 25) and it just hangs or returns a timeout error, that’s a strong sign port 25 is inaccessible from your network.

4. ISP Blocks or Network Restrictions

Some ISPs silently block port 25 without notification. This is especially common on home or non-business connections. In such cases, you’ll need to check if port 25 is open manually or switch to an alternative like port 587.

If you notice any of these red flags, it’s time to test your connection directly.

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How to Check if Port 25 Is Open in Windows (Including Windows 10)

If you’re using a Windows system and want to find out whether port 25 is open, there are several methods available, ranging from built-in tools like Telnet to more modern approaches using PowerShell or online checkers. Below are the most reliable ways:

i. Using Telnet (All Windows Versions Including Windows 10)

Step 1: Enable Telnet (Windows 10 only)

Telnet isn’t installed by default on Windows 10, but you can enable it easily:

• Open Control Panel → Programs and Features
  
• Click Turn Windows features on or off
  
• Check Telnet Client, then click OK

Step 2: Open Command Prompt

• Press Windows + R, type cmd, and press Enter

Step 3: Run the Telnet Command

bash

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telnet smtp.domain.com 25

Replace smtp.domain.com with the actual domain or IP of the mail server.

What You Should See:

• If port 25 is open, you’ll get a response like:
  220 smtp.domain.com ESMTP Postfix
  
• If it’s blocked, the command will hang or show a timeout error.

This is the classic method when you’re looking up how to check if port 25 is open in Windows.

ii. Using PowerShell

If Telnet feels outdated, PowerShell provides a quick way to test connections:

powershell

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Test-NetConnection -ComputerName smtp.domain.com -Port 25

Interpret the Result:

• TcpTestSucceeded : True means the port is open.
  
• False means it’s blocked or unreachable.

This is a cleaner and more scriptable method, especially useful in automated testing or scripts.

iii. Using Online Port Checkers on Windows

If you don’t want to use any command-line tool:

• Go to https://canyouseeme.org
  
• Enter port 25 in the input box
  
• Click Check Port

This tool tests your public IP’s port 25 status. It’s handy for checking whether your ISP is blocking port 25.

SEE ALSO: Cryptography Vs Cybersecurity: Importance of Data Security

How to Check if Port 25 Is Open in Linux

For Linux users, especially system admins running servers, knowing whether port 25 is open can save hours of email troubleshooting. Thankfully, Linux provides multiple tools to get this done efficiently, even from the terminal.

i. Using Telnet on Linux

Step 1: Open Terminal

Step 2: Run the Command

bash

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telnet smtp.domain.com 25

If Telnet is not installed:

bash

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sudo apt install telnet      # Ubuntu/Debian  

sudo yum install telnet      # RHEL/CentOS  

Expected Output:

• Success: 220 smtp.domain.com ESMTP
• Failure: hangs, “connection refused,” or “unable to connect”

This is one of the fastest answers for how to check if port 25 is open Linux or how to check SMTP port is open or not in Linux.

ii. Using Netcat (nc)

Netcat is a lightweight utility that’s perfect for checking port status.

Command:

bash

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nc -zv smtp.domain.com 25

Output Interpretation:

• succeeded = port is open
  
• connection refused or timed out = port is blocked or closed

You can also check your localhost:

bash

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nc -zv localhost 25

Netcat is often preferred over Telnet in modern Linux systems because it’s faster and more versatile.

iii. Using Nmap for a Deeper Scan

If you want a broader scan with more context:

Install Nmap:

bash

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sudo apt install nmap

Command:

bash

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nmap -p 25 smtp.domain.com

Output Example:

arduino

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PORT   STATE  SERVICE

25/tcp open   smtp

This tells you clearly whether port 25 is open and what service is running on it.

These three tools, Telnet, Netcat, and Nmap, give you everything you need when wondering how to check if port is open Linux, especially for SMTP services.

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How to Check if Port 25 Is Open on macOS

If you’re on a Mac and trying to send emails through an external SMTP server, checking whether port 25 is open is a crucial troubleshooting step, especially if your mail client keeps failing to send.

macOS is Unix-based, which means you have access to the same powerful networking tools as Linux.

i. Using Telnet via Terminal

Step 1: Open Terminal

You can find it via Spotlight (Cmd + Space → type “Terminal”) or go to:
Applications → Utilities → Terminal

Step 2: Run the Command

bash

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telnet smtp.domain.com 25

Response You Want:

pgsql

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220 smtp.domain.com ESMTP Service Ready

No Response or Timeout:

That means port 25 is likely blocked, either by your ISP, firewall, or the destination server.

Telnet is still available on most macOS versions. However, if it’s missing, consider using Homebrew to reinstall it.

ii. Using Netcat on macOS

Netcat comes pre-installed on macOS, so this is often the faster method:

bash

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nc -zv smtp.domain.com 25

Output Example:

• Success: Connection to smtp.domain.com port 25 [tcp/smtp] succeeded!
  
• Failure: Connection refused or timed out

This is a quick solution for those wondering how to check if port 25 is open mac without needing to install extra tools.

ii. Bonus: Check Local Firewall Settings

Sometimes the issue isn’t external, it’s your Mac’s firewall.
Go to:

• System Preferences → Security & Privacy → Firewall
• Ensure that outgoing connections aren’t being restricted.

In most cases, these two tools, Telnet and Netcat, will give you a clear answer and help you determine whether SMTP over port 25 is being blocked on macOS.

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How to Check If Port 587 Is Open Without Telnet

Port 587 is the recommended alternative to port 25 for sending emails, especially when authentication is required. Most modern email providers (like Gmail, Outlook, and Zoho) use port 587 for SMTP submission. If you suspect port 25 is blocked, port 587 is often your best workaround.

But what if Telnet isn’t available, or you’d rather not use it? Here’s how to check if port 587 is open without Telnet, using practical tools built into most systems.

i. Using Netcat (nc)

Netcat is one of the simplest ways to test if port 587 is open, and it’s available by default on most macOS and Linux systems (and installable on Windows via tools like Git Bash or WSL).

Command:

bash

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nc -zv smtp.domain.com 587

Expected Output:

• Success: Connection to smtp.domain.com port 587 [tcp/message submission] succeeded!
• Failure: Connection timed out or refused

This is fast, scriptable, and reliable, perfect for checking connectivity without Telnet.

ii. Using OpenSSL (for Secure SMTP Testing)

SMTP over port 587 typically uses STARTTLS to encrypt the connection. You can simulate this with OpenSSL:

Command:

bash

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openssl s_client -starttls smtp -connect smtp.domain.com:587

Success Output:

A long TLS handshake followed by SMTP server greetings like:

pgsql

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220 smtp.domain.com ESMTP Ready

If the port is blocked, the command will hang or return an error about connection failure.

This method is especially useful when you’re working in a secure environment and need to ensure that SMTP authentication and encryption are working properly.

iii. Using Nmap for Port 587

Another non-Telnet method is Nmap. While it’s often used for broader scanning, it works great for checking single ports:

bash

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nmap -p 587 smtp.domain.com

Look for:

arduino

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587/tcp open  submission

That’s your green light.

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Using Online Port Checkers (For Any OS)

If command-line tools feel too technical, or if you’re troubleshooting from a restricted device, online port checkers offer a fast and user-friendly way to test whether port 25 (or any SMTP port) is open from the public internet.

These tools are especially helpful for checking if your ISP or firewall is blocking outbound SMTP traffic, even when the server itself is configured correctly.

i. Canyouseeme.org

One of the most widely used tools for testing port accessibility.

Steps:

1. Visit https://canyouseeme.org
   
2. Enter 25 in the port field.
   
3. Click Check Port.

Results:

• If successful: “Success: I can see your service on [Your IP] on port (25)”
• If blocked: “Error: I could not see your service…”
  

Works for checking port 25, 587, or any other port.

ii. MXToolbox SMTP Test

Go to https://mxtoolbox.com/diagnostic.aspx

Use the “SMTP Test” tool to:

• Input your mail server (e.g., mail.yourdomain.com)
• Run an SMTP diagnostic check

It tests both port accessibility and SMTP handshake behavior, helping you isolate whether the issue is with the network or SMTP configuration.

iii. YouGetSignal

Another clean and simple checker: https://www.yougetsignal.com/tools/open-ports/

Steps are similar:

• Enter IP/hostname
  
• Enter port 25 or 587
  
• Click “Check”

Pros of Online Port Checkers:

• No need to install tools or run terminal commands
  
• Great for verifying ISP-level blocks
  
• Platform-independent (works on Windows, Mac, Linux, even mobile)
  

Limitations:

• Only checks from the public internet, not internal LAN connections
  
• Won’t show firewall rules or local blocking unless port forwarding is in place

These tools give you a reliable, zero-effort way to quickly answer the question: “Is port 25 open from the outside world?”

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What to Do If Port 25 Is Blocked

So, you’ve run the tests, and all signs point to port 25 being blocked. Now what? Here’s what you can do to get your emails flowing again without hitting a brick wall.

1. Use an Alternative SMTP Port (587 or 2525)

Most modern SMTP servers support other ports alongside 25:

• Port 587 is the standard for email submission with authentication and encryption.
  
• Port 2525 is often supported by transactional email providers like SendGrid, Mailgun, and Postmark as a backup.

Update your mail client’s settings or application code to use port 587 or 2525, and include STARTTLS or SSL/TLS if required.

This workaround often bypasses ISP restrictions and ensures your email still goes through securely.

2. Contact Your ISP or Hosting Provider

If you’re managing your own server (on a VPS, for example), contact the host’s support team and ask:

“Is port 25 open on my instance? If not, can it be unblocked or do you offer email relay?”

Some cloud providers like AWS and Google Cloud block port 25 by default but allow you to request an unblock, provided you explain your use case.

3. Use a Smart Host or SMTP Relay Service

If your ISP blocks port 25 and won’t open it, SMTP relay services can help. These act as a middleman between your application and the recipient’s server.

Popular options:

• Amazon SES
  
• SendGrid
  
• Mailgun
  
• SMTP2GO

You configure your server or application to send email through the relay using port 587 or 465, and they handle the rest.

4. Check Local Firewall or Router Settings

Sometimes the problem is on your end:

• Ensure your local firewall isn’t blocking outbound connections on port 25.
  
• On networks with routers/firewalls, make sure port 25 is open in the outbound direction (note: port forwarding is not relevant here unless you’re hosting a server).

5. Avoid Using Port 25 for Authenticated Email Clients

Port 25 is not intended for email submission from authenticated users, use 587 instead. Even if it’s unblocked, using port 25 for client-to-server mail can result in issues or spam filtering.

In short, you don’t always need port 25 open to send emails. By switching to alternative ports or services, you can maintain deliverability and stay compliant with modern email standards.

Important Cautions and Legal Considerations

Before you start checking ports, especially on remote servers, it’s important to understand the boundaries of what’s legal and ethical in port scanning and network testing.

Here are some key cautions:

1. Always Have Permission Before Scanning External Systems

Running tools like Nmap or Netcat on servers you don’t own can be flagged as suspicious or even malicious behavior. Port scanning is sometimes associated with hacking attempts, and in some jurisdictions, doing so without consent could be illegal.

Only test ports on systems you manage or have been authorized to assess.

2. Respect ISP and Cloud Hosting Policies

Most Internet Service Providers block port 25 on residential plans, and trying to circumvent these restrictions could violate their terms of service. If your provider doesn’t support unblocking port 25, switch to a compliant SMTP provider instead of trying to bypass it.

Cloud hosts like AWS, Google Cloud, and Azure also have strict rules about using port 25. You often need to submit a formal request to have it unblocked, and even then, they’ll require details on how you’ll prevent abuse (e.g., no spamming).

3. Be Careful with Firewall Configuration Changes

Opening up port 25 on your server without proper access controls, authentication, or spam protection could turn your server into an open relay, which can quickly get your domain or IP blacklisted for spam.

Always:

• Enable SMTP authentication
  
• Use TLS encryption
  
• Restrict access if necessary

4. Avoid Open Relays

An open SMTP relay allows anyone on the internet to send emails through your server. This is a massive security risk and a fast track to being blacklisted.

Use SMTP authentication, whitelisted IPs, and strict relay rules to stay protected.

Taking a few precautions now can help you avoid blacklisting, service bans, or security breaches later.

Final Thoughts

Knowing how to check if port 25 is open is a foundational skill for anyone managing email services, troubleshooting delivery issues, or securing server infrastructure.

From using Telnet and Netcat on Windows, Linux, or macOS, to leveraging online tools and port scanners, there are multiple ways to test SMTP connectivity. And if port 25 is blocked, you’re not stuck. Switching to port 587, using a smart SMTP relay, or contacting your hosting provider can restore seamless email delivery.

Whether you’re trying to figure out how to check if port 25 is open in Windows 10, running commands to check if SMTP port is open or not in Linux, or just exploring how to check if port 25 is open SMTP-style, one truth remains: visibility into your port access is key to solving problems before they escalate.

In a world where email is critical, don’t let something as small as a blocked port stop your communication.

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