If your emails aren’t getting delivered, the problem often starts with SMTP—the protocol that handles outgoing email. You might think things are working fine because your software doesn’t show errors. But without SMTP testing, you won’t know if your server is misconfigured, if encryption is failing, or if spam filters are silently blocking your messages.

SMTP testing tools help you identify and fix these issues before they lead to email deliverability issues or security concerns. They simulate how your server sends emails and how receiving servers respond to it.

This article breaks down SMTP testing, explains how it works (including with manual commands), and covers 11 SMTP testing tools that make this process easier. We will also understand how you can test your emails’ deliverability using SmartDelivery placement tests. 

Let’s get started.

What is SMTP Testing?

SMTP testing is the process of verifying whether your email server is properly set up to send messages. It checks if your server accepts connections, authenticates users, supports encryption protocols like STARTTLS, and handles errors correctly.

A full SMTP test usually includes:

- Verifying server availability on ports like 25, 587, or 465

- Checking for correct SMTP command responses (HELO, MAIL FROM, RCPT TO, etc.)

- Confirming that authentication is required

- Ensuring STARTTLS or SSL is supported for secure delivery

- Testing if the server is an open relay

- Validating email headers and domain records like SPF, DKIM, and DMARC

By running these checks, you ensure that your server is both functional and secure.

Why Do You Need to Do an SMTP Test?

Sending emails without testing your SMTP setup is like driving without checking your tires or fuel. 

While everything might seem fine on the surface, you’re running the risk of a serious breakdown when it matters most. 

SMTP testing serves as a crucial step to ensure that your emails are not only getting delivered but also reaching their destination securely and reliably. 

Here's why it's so important:

Ensure Your Emails Reach Their Destination

SMTP testing ensures that your email server is properly configured to send and receive messages. 

Without this check, your emails might end up in the wrong place, like the spam folder, or fail to be delivered altogether. 

By confirming that your server is set up correctly, you make sure that your emails reach the inboxes of your recipients—smoothly and without issues. This is vital for maintaining consistent communication with your audience, especially if you send cold emails.

Protect Your Sender Reputation

Your reputation as a sender is one of the most important factors in ensuring email deliverability. 

If your server is misconfigured, you risk being flagged by ISPs as a source of spam. 

Once your domain is blacklisted, your emails are more likely to be blocked or marked as spam. Regular SMTP tests help ensure that your configuration is in line with best practices and that your reputation stays intact. 

This proactive measure helps you avoid losing your email sender status, which is crucial for ensuring your messages are trusted and welcomed by your recipients.

Prevent Unauthorized Access to Your Server

SMTP testing can help prevent malicious actors from hijacking your email server. 

Spammers often take advantage of unsecured servers to send bulk emails, which can lead to your IP being blacklisted. By testing your server’s security settings, you can make sure only authorized users are able to send emails through it. This not only prevents unauthorized use of your server but also helps protect your brand from the negative effects of being associated with spam.

Fix Configuration Problems Before They Cause Issues

A common cause of email delivery failures are simple configuration errors—things like incorrect DNS settings or blocked ports. These issues can stop your emails from reaching their destination or even prevent them from sending at all. 

SMTP tests allow you to pinpoint these configuration problems and fix them before they cause problems with email delivery. By conducting tests regularly, you catch errors early, making sure your emails continue to flow without disruption.

Ensure Secure Email Transmission

Email security is essential, especially when sensitive information is involved. SMTP testing checks that your server is using proper encryption protocols, such as SSL/TLS, to protect your emails during transmission. 

Without encryption, your emails are vulnerable to being intercepted or tampered with. This puts your data and your recipients' data at risk. By running regular SMTP tests, you ensure your email communications are secure, helping prevent email spoofing or other forms of unauthorized access.

Maintain Consistency in Communication

Reliability is key when it comes to email communication. If emails are delayed, fail to send, or end up in spam, it can frustrate your recipients and damage your brand’s credibility. SMTP testing helps ensure that your emails are consistently delivered to the right places and on time. 

SMTP tests help maintain that reliability by flagging any issues that could disrupt the delivery process.

Troubleshoot Problems Efficiently

SMTP tests don’t just identify issues; they provide detailed reports that help you understand exactly what went wrong. 

Whether it’s an issue with authentication or a problem with the server configuration, these reports give you the information you need to troubleshoot effectively. Instead of waiting for customers to report email failures, you can proactively address potential problems. 

Meet Industry Compliance Standards

For businesses in regulated industries, compliance with email security standards is a must. SMTP testing ensures that your email server meets the required security standards, such as GDPR, HIPAA, or PCI-DSS, depending on the nature of your business. 

If you handle sensitive customer data, it’s vital to ensure that your emails are being transmitted securely and in accordance with relevant regulations. SMTP testing helps you stay compliant, minimizing the risk of legal penalties and ensuring that your customers' information is protected.

Types of SMTP Tests

SMTP testing covers a variety of checks to ensure your email server is configured correctly, securely, and is capable of successfully delivering emails. 

Here are the essential tests that should be conducted regularly to maintain your email server's performance:

1. Connectivity Check

This test verifies whether your SMTP server is reachable over the network and is listening on the correct port. 

A failed connectivity check could indicate that your server is down, your port settings are misconfigured, or your server is behind a firewall preventing access. 

2. Authentication Test

SMTP authentication is vital for preventing unauthorized users from using your server to send emails. 

This test verifies whether your server requires login credentials and which authentication methods (e.g., LOGIN, PLAIN, CRAM-MD5) are supported. 

3. TLS/SSL Encryption Test

This test ensures that your SMTP server supports encrypted connections, either through STARTTLS or SSL. 

Encryption is critical for securing the data transmitted between your server and the recipient’s server, ensuring that sensitive email content cannot be intercepted during transit. 

4. Open Relay Test

An open relay allows unauthorized users to send emails through your server, making it an attractive target for spammers. 

This test checks whether your SMTP server allows open relays and ensures that it only accepts email submissions from authorized sources. 

5. Command Response Validation

This test simulates the SMTP command sequence and checks whether the server responds correctly at each stage of the process. 

It ensures that the server follows the proper communication protocols and that all commands are executed as expected. 

6. Header and DNS Record Validation

This test checks the configuration of your email headers and DNS records (like SPF, DKIM, and DMARC) to ensure that they are properly set up and aligned with your domain’s security policies. 

Valid DNS records help protect your domain from email spoofing, phishing, and other malicious activities by verifying that your emails are coming from a trusted source.

7. Delivery Testing

Delivery testing is perhaps the most crucial aspect of SMTP testing. 

It checks whether your email successfully reaches the recipient’s inbox or if it gets caught in spam filters, rejected, or bounced back. 

This test helps identify issues such as incorrect configuration settings, DNS record problems, or blacklisted IP addresses that could prevent emails from being delivered properly. SmartDelivery (by Smartlead) is an email placement tool that helps you see where your emails are landing, check the spam scores, and associated sender accounts for any blacklists or IP issues. (more on this later)

How to Perform SMTP Testing: A Step-by-Step Guide

To perform successful SMTP testing, you need to configure your SMTP server properly. 

This involves gathering the necessary details, such as the server address, port number, security settings, and authentication credentials. 

Here’s a simple guide on how to go about the setup and testing process:

Step 1: Gather SMTP Configuration Details

Before you start, obtain the configuration details for your SMTP server. These are usually provided by your email service provider or SMTP delivery service and include the server address, port number, security settings, and authentication details.

Step 2: Access SMTP Settings

Open your email client or an SMTP testing tool of your choice. Navigate to the SMTP settings section, where you'll enter the configuration information.

Step 3: Enter the Server Address

In the SMTP settings, enter the server address that you’ve obtained from your provider. This is usually a domain name like smtp.yourprovider.com.

Step 4: Specify the Port Number

Next, specify the correct port number for SMTP communication. Common SMTP ports are 25, 465, or 587, depending on the type of connection (unencrypted or encrypted) your server requires.

Step 5: Enable Security Protocols

If your server requires a secure connection, enable the appropriate security protocols, such as TLS or SSL. These protocols ensure that your emails are transmitted securely, protecting sensitive information.

Step 6: Enter Sender and Recipient Email Addresses

To simulate sending a test email, input both the sender's and recipient's email addresses. This will allow you to verify that the email delivery process works smoothly.

Step 7: Check Secure Connection Option

If your server needs a secure connection, make sure to check the option for a secure connection. This ensures that the test email is sent using the encryption settings you’ve specified.

Step 8: Provide Authentication Credentials (If Required)

If the SMTP server requires authentication, enter the username and password provided by your email provider. This step ensures that only authorized users can send emails through the server.

Step 9: Run the SMTP Test

Click the "Test SMTP" button or equivalent in your email client or tool to begin the test. The tool will attempt to connect to the server and simulate an email sending process.

Step 10: Review Test Results

Once the test is complete, review the results carefully. Check for successful connections, email delivery, and any error messages or issues that may arise. Common issues to look for include connection timeouts, authentication errors, or incorrect port settings.

Common SMTP Commands

1. HELO/EHLO

Command Form: HELO <domain> or EHLO <domain>

Description: Initiates the conversation between the client and server. EHLO also requests the server to provide a list of extended features it supports.

2. MAIL FROM

Command Form: MAIL FROM: <reverse-path>

Description: Specifies the sender’s email address.

3. RCPT TO

Command Form: RCPT TO: <forward-path>

Description: Specifies the recipient’s email address.

4. DATA

Command Form: DATA

Description: Indicates that the message body is about to be sent, followed by the email content.

5. RSET

Command Form: RSET

Description: Resets the session, clearing all recipient addresses and any other state information.

6. VRFY

Command Form: VRFY <string>

Description: Requests verification of an email address. The server will respond with whether the email address is valid.

7. EXPN

Command Form: EXPN <mailing-list>

Description: Asks the server to expand a mailing list, listing all individual addresses within the list.

8. HELP

Command Form: HELP <string>

Description: Requests help or a list of available commands from the server.

9. NOOP

Command Form: NOOP

Description: A "no-operation" command that keeps the connection alive, typically used to maintain the session.

10. QUIT

Command Form: QUIT

Description: Ends the SMTP session gracefully, signaling that the conversation is complete.

11. TURN

Command Form: TURN

Description: Switches the roles of the client and the server. It is not commonly used.

12. AUTH

Command Form: AUTH <mechanism>

Description: Initiates an authentication process, allowing the client to provide login credentials (e.g., LOGIN, PLAIN, CRAM-MD5).

13. STARTTLS

Command Form: STARTTLS

Description: Initiates a secure encrypted connection using SSL/TLS to protect the communication.

14. ATRN

Command Form: ATRN <domain>

Description: Allows a mail relay on demand, letting the client request the mail relay to send mail on behalf of the domain.

15. SOML

Command Form: SOML FROM: <reverse-path>

Description: Sends the message to the recipient if they are currently connected; otherwise, it sends it to their mailbox.

16. SEND

Command Form: SEND FROM: <reverse-path>

Description: Similar to SOML, but only sends the message if the recipient is currently connected.

17. SAML

Command Form: SAML FROM: <reverse-path>

Description: Sends the message to the recipient if they are connected, or stores the message for later retrieval.

18. PIPELINING

Command Form: PIPELINING

Description: Allows the client to send multiple commands in a single transmission, improving performance.

19. SIZE

Command Form: SIZE <size-in-bytes>

Description: Specifies the maximum allowed size for an email message. This command is used to check whether an email will be accepted by the server.

20. DELIVERBY

Command Form: DELIVERBY <date-time>

Description: Requests that the message be delivered by a specified date and time. Not widely supported.

11 Best SMTP Testing Tools

1. MXToolbox

MXToolbox offers a free SMTP diagnostics tool that lets you check open relay status, test TLS encryption, and inspect headers. It also checks blacklist status and offers DNS lookups for SPF, DKIM, and DMARC.

You enter your domain or IP, and it gives you a step-by-step breakdown of how your server responds to SMTP commands. It’s fast and reliable for both one-off tests and ongoing monitoring.

2. Mailtrap

Mailtrap creates a fake SMTP environment where you can test emails without sending them to real users. It captures the entire SMTP transaction, shows headers, spam score, and lets you debug your email code safely.

It’s ideal for developers who need to validate email logic before going live. Mailtrap also lets you run authentication and TLS checks during development.

3. CheckTLS

CheckTLS tests your server’s ability to send and receive encrypted email. You can simulate email sending between your domain and another to see if TLS is supported end-to-end.

It also includes full email delivery diagnostics, header analysis, and DNS record checks. This is great if you’re in a regulated industry where encryption is non-negotiable.

4. Wormly

Wormly’s SMTP test tool is quick and no-login. Enter your server details, and it checks command responses, TLS support, and authentication enforcement.

It’s useful for basic sanity checks if you don’t want a full platform. Plus, it gives you clear response codes and errors in one view.

5. SocketLabs SMTP Diagnostic Tool

This tool tests SMTP credentials, host and port configuration, and encryption. You get a detailed log of the send attempt, including where and why it failed.

SocketLabs also provides real-time feedback on blacklist status and spam triggers. It’s useful if you manage email marketing at scale and want deep diagnostics.

6. SSL Labs (Qualys)

Though not built solely for SMTP, SSL Labs can scan your mail server’s SSL/TLS setup. It checks for weak ciphers, expired certificates, and outdated protocols.

Run this test to verify that your encrypted emails aren’t exposed to downgrade attacks or poor security practices.

7. TestSMTP.com

This tool gives you full control over the SMTP session. You manually enter commands (HELO, MAIL FROM, etc.) and see raw server responses.

It’s excellent for advanced users who want to test edge cases, handle non-standard behaviors, or debug complex SMTP issues.

8. Microsoft SMTPDiag Tool

This Windows utility is designed for Exchange and Office 365 environments. It checks SMTP communication paths, DNS resolution, MX record routing, and login permissions.

For teams using Microsoft infrastructure, this is a go-to tool for internal SMTP issue tracking.

9. Pingability SMTP Monitor

Pingability focuses on uptime. It runs regular SMTP checks across global servers and notifies you if your server is down, slow, or misconfigured.

It’s great for ongoing monitoring and helps DevOps teams get alerts before users report issues.

10. Mailgun SMTP Logs + Test Mode

If you’re using Mailgun to send transactional emails, its SMTP logs show full delivery paths, retry attempts, and bounce responses.

It also includes a test mode where you can simulate a full send cycle without actually sending emails. Ideal for API-driven applications.

11. Paubox SMTP Test

Paubox offers encrypted email services and its SMTP test checks whether your outbound emails use secure TLS channels.

It’s especially suited for healthcare or finance businesses that deal with sensitive data. The tool also validates headers and confirms HIPAA-safe delivery paths.

Deliverability Testing Using SmartDelivery (by Smartlead)

SMTP testing isn't limited to verifying server configurations; it also includes checking whether your emails are actually reaching your recipients' inboxes. 

This is where deliverability testing comes in, and SmartDelivery provides a reliable solution to automate and streamline this process. 

Here’s how to run a SmartDelivery test:

Step 1: Log In to Your Smartlead Account

To get started, you'll first need to log into your Smartlead account. If you don’t have one, sign up for a trial account. 

After logging in, you need to navigate to the dashboard where you can access SmartDelivery.

Step 2: Choose Test Mode - Manual or Automatic

Once in SmartDelivery, you'll have the option to choose between manual and automatic testing. 

Select the "Manual Test" option. You can also initiate a test by clicking the "Start Spam Test" button located at the top right corner of the screen.

Step 3: Enter General Test Details

Next, you’ll need to fill in some basic information to organize your test:

- Test Name: Name your test so you can easily distinguish it from others.

- Description: Provide a short description of the test.

- Move to a Folder: Organize your tests by placing them into relevant folders (optional).

At this stage, the test will run through the SpamAssassin spam filter, which evaluates your email against various spam detection criteria. The SpamAssassin filter scores your email based on factors like content quality, sender reputation, and technical setup.

Additionally, you can enable the Link Checker to ensure that all links (except the unsubscribe link) in your email are working correctly. You can also set the sending throttle rate, which controls how quickly emails are sent to the seed list—either all at once or at specific intervals (e.g., one email every minute).

Once you’ve filled out these details, click Save & Next to continue.

Step 4: Select Mailboxes and Email Copy

In this section, you will configure the mailboxes and email copy for your test:

- Campaign Selection: From the dropdown, select the campaign you want to test. This will allow you to choose a sequence (the email content) for that campaign.

- Preview Email: You can preview the email sequence to ensure that the correct copy is selected.

Now, decide whether you want to test using a Smartlead Connected Mailbox (an account already added to your Smartlead account) or a non-connected mailbox. For this guide, we’ll use the connected mailbox option, as it provides more detailed results.

If you choose to test using a connected mailbox, you can select the sender account from the campaign's sender list.

Additionally, you have the option to Modify Email Providers. This allows you to send the test to a seed list, which includes over 50 email addresses from different email service providers (ESPs). This gives you insights into how your email performs across multiple ESPs worldwide. We recommend selecting all available email providers for the most comprehensive results.

Step 5: Run the Test

After reviewing your settings, click Save & Run Test. SmartDelivery will send your email to the seed list, testing how it performs across different providers. This includes assessing whether your emails pass through spam filters, if they’re flagged for blacklisting, and how they perform in terms of deliverability.

Once the test is complete, you will receive a detailed deliverability report. This report highlights any issues or challenges your email might face and provides suggestions for improvement.

You can learn more about SmartDelivery here. 

Final Thoughts

SMTP testing isn’t just a technical checkbox. It’s a core part of your email delivery strategy. Whether you’re sending cold emails, newsletters, or password resets, SMTP issues can ruin your reach or expose your domain to abuse.

Start with basic testing using SMTP testing tools like MXToolbox or Wormly. For deeper visibility, try Mailtrap, CheckTLS, or Mailgun. If you need a security-focused approach for compliance-heavy systems, Paubox or SSL Labs are worth considering.

And if you're looking for an end-to-end deliverability test—complete with inbox placement, spam filter checks, and authentication analysis—SmartDelivery is a strong option. It includes free tests and helps you monitor how your campaigns perform across global inbox providers.