SMTP is an acronym for Simple Mail Transfer Protocol—the protocol used for sending emails.
One of the problems that can occur when SMTP isn’t functioning correctly is something called SMTP Error 501. Primarily a syntax error, SMTP Error 501 actually stops emails from being sent and/or received.
Usually, senders receive a bounced back email stating that their email could not be sent due to SMTP Error 501—which can be incredibly frustrating and disruptive to business operations.
This guide goes over what SMTP Error 501 is, what causes it, and how to fix it so your emails can be transmitted smoothly and seamlessly.
SMTP Error 501 generally occurs due to an incorrectly formatted email address or SMTP command lines when trying to send an email.
- The error contains “501” somewhere in the body of the response message with accompanying text such as “Syntax error in parameters or arguments” or “Invalid address.”
- This focuses exactly on what the issue is, whether an SMTP command is improper, whether there were no angle brackets surrounding the email address, or whether the email address itself is erroneous.
In layman’s terms, the server rejects the request due to bad syntax and subsequently issues an SMTP error code pertaining to the incorrectly written email command or the incorrectly formatted email address.
SMTP Error 501 is caused due to many reasons and mostly syntax errors and formatting errors. Here are some common triggers:
- Incorrect email format. The simplest reason for SMTP Error 501 is that you entered an email address that does not follow the standard format (such as missing up the “@” sign or domain part).
- SMTP command syntax errors. This causes an error when the email client sends SMTP commands to the server, and they’re not in the correct format. Examples include: missing parameters, incorrect spacing, and unsupported commands.
- Configuration settings. For example, if an email client is misconfigured — if its SMTP server details are incorrect — it can cause syntax errors which results in a 501 error.
- Case sensitivity in email addresses. If the email address contains special characters not supported by the email server, it might consider the address invalid and return a 501 error.
- Incomplete or extra spaces. A trailing whitespace on an email address, or extra key spaces (or missing information) on the command line gets misinterpreted by the server, which flags it as a syntax error.
- Verify email formats. Double-check the recipient’s email address for any formatting errors. Ensure it follows the standard format (
username@example.com). - Check settings. Go to Settings > See all settings > Accounts and Import to review the “Send mail as” settings, ensuring the email address is correctly formatted and the SMTP server settings are accurate.
- Account configurations. In Outlook, go to File > Account Settings > Account Settings. Select your email account and click “Change” to verify your outgoing mail server (SMTP) settings are correct. Also, ensure the email address is in the correct format.
- Email syntax. Before sending an email, review the recipient’s email address for any typographical errors or syntax issues that could lead to a 501 error.
- Precautions and settings. Similar to Gmail and Outlook, verify the email address format and SMTP server settings within the account settings section of your email service. Ensure there are no extra spaces or special characters in the email addresses.
- Service-specific guidelines. Consult the help section or support center of your email service for any specific instructions on preventing SMTP Error 501, as some services might have unique requirements.
- Command syntax checks. When configuring email servers or sending commands directly, ensure that SMTP command lines are correctly formatted. Refer to the SMTP protocol documentation for guidance.
- Server configurations. Regularly review server settings, particularly those related to email sending (e.g., SMTP relay settings). Ensure your server’s IP address is not listed on any blacklists, as this can indirectly cause SMTP errors.
- Logging and monitoring. Implement logging of SMTP transactions. These logs can provide insights into any syntax errors or misconfigurations leading to SMTP Error 501.
- Test environments. Before making changes in a live environment, use test email accounts to send messages and confirm that no syntax errors occur, particularly when updating server configurations or deploying new email policies.
📝 Tip: Regularly use Warmy’s free email deliverability test as this gives insights into possible issues.
Understanding these variants helps pinpoint the specific problem for more targeted resolutions.
Implications. This variation often points to issues with the command syntax, possibly due to incorrect spacing or unsupported characters in the email header.
Resolution strategies. Review the syntax of your SMTP commands closely. Ensure you’re following protocol specifications for command format and spacing. Remove any unsupported characters from the email header.
Implications. Indicates a problem with the sender’s address, often related to incorrect domain formatting or the use of prohibited characters.
Resolution strategies. Double-check the sender’s email address for any formatting errors. Verify that the domain name is correctly spelled and does not include illegal characters. Ensure the address matches the domain you are authorized to send from, particularly if DMARC policies are in place.
Implications. Similar to 5.1.7, this code typically means there’s a syntax error in the recipient’s email address, such as missing “@” or other formatting issues.
Resolution strategies. Confirm the recipient’s email address for correct syntax, ensuring it includes the “@” symbol and a valid domain. Advise recipients to check their email settings if their addresses are consistently causing this error.
Implications. This variant usually signifies a failure due to authentication issues, where the sender is not authorized to send emails from the specified domain or IP.
Resolution strategies. Check your SPF and DKIM records to ensure they’re correctly configured, allowing you to send emails from your domain. Review any domain-based message authentication, reporting, and conformance (DMARC) policies to confirm they’re not restricting your ability to send mail.
🔖 Related Reading: SPF, DKIM, and DMARC: Boosting Email Security and Deliverability
Using Warmy.io can help improve sender reputation and reduce SMTP problems, which are important for keeping email operations running smoothly.
To create a favorable sending history, these services progressively increase the number of emails sent from a new account or domain, imitating regular email activity.
Warmy.io’s warmup process advantages include:
👉 Enhances sender reputation. Warmy.io sends emails from your account in a methodical manner, progressively turning up the volume over time. By doing this, you can improve your sender reputation by making sure that email providers and ISPs view your account as a reliable sender.
👉 Minimizes SMTP faults. The chance of running into SMTP faults such as the 501 error is decreased when a sender has a strong reputation. Unexpected jumps in email volume might raise red flags for email providers, but services like Warmy.io make sure your emails are less likely to be rejected or marked as spam.
👉 Boosts email deliverability. Warmy.io helps make sure your emails get through to the mailbox instead of being filtered as spam or bouncing back from SMTP issues by enhancing your sender reputation and reducing mistakes. Whether you are sending transactional, marketing, or personal emails, this is essential for efficient email communication.
👉 Saves time and resources. Setting up and carrying out an email sending schedule takes a lot of work when developing a sender reputation manually. Warmy.io streamlines this procedure, freeing you up to concentrate on content creation and improved email management.
👉 Gives you useful insights. Warmy.io does more than just warm up your emails; it also provides you with analytics and feedback on how well your emails are performing. You can use this information to improve your email strategy and lower the frequency of SMTP problems by learning more about open rates, bounce rates, and other important metrics.
Additional warmup features that enhance deliverability
Aside from the automated warmup process, Warmy has other tools and features. When used in parallel to the warmup, email deliverability can drastically improve.
Free email deliverability test and Domain Health Hub
The free email deliverability test from Warmy provides a comprehensive assessment of your emails and where they are landing. Here are some other examples of the valuable information and data the deliverability test provides:
- Inbox placement analysis: the percentage of your emails that end up in the spam folder, promotions tab, inbox, and even the unreceived ones.
- Blacklist status: Learn if your domain or IP is listed on any blacklists so you can proceed with the delisting process.
- Authentication verification: Assess your email reputation and verifies your email authentication settings such as SPF, DKIM, and DMARC.
Additionally, Warmy’s Domain Health Hub is on a level of its own. Instead of monitoring individual email addresses, Warmy.io provides a domain-level health dashboard so businesses can take a more strategic approach to email deliverability. The Domain Health Hub includes the following features:
- A domain health score based on a combination of various factors like authentication, blacklist status, and inbox placement tests. You’ll also be able to monitor your spam rate trends and overall deliverability performance with weekly or monthly tracking options.
- Comprehensive DNS status checks to easily validate SPF, DKIM, DMARC, rDNS, MX, and A records for stronger authentication & security.
- Optimized multi-domain monitoring so users can manage all their domains from one dashboard and identify which ones need immediate attention.
- Quick access to a detailed breakdown of health metrics, performance reports, and deliverability trends per domain.
SPF, DKIM, and DMARC testing
Automatically validate and optimize SPF and DMARC records with Warmy’s built-in authentication checker and free record generators (for SPF and DMARC) to improve email security and deliverability.
Advanced email warmup customization:
Tailor your warmup distribution across various email providers (Gmail, Outlook, etc.) and define engagement patterns for better warmup performance.
You can also tailor engagement patterns by choosing between B2B or B2C customers.
Established email seed list
Benefit from real engagement by using Warmy’s advanced seed list, with automated management and API integration for optimized inbox placement and stronger sender reputation.
Warmy has also recently introduced the API Endpoint for Established Seed List—a new way to access, manage, and configure your seed list splits directly from your system. This means:
- Seamless integration with real-time seed list data that goes straight into your system with API access.
- Efficiency boost via automated split management which reduces errors, and saves valuable time.
- Better inbox placement due to properly configured splits mean fewer bounces and stronger sender reputation.
Learn more about Warmy’s seed lists here. Or, you may want to get a more in-depth look at how Warmy’s seed lists work and what they can do for your email deliverability:
Ultimately, the best way to avoid SMTP Error 501 in the future is to make the required changes to email address syntax and SMTP commands formatting immediately.
Preventative solutions involve awareness of email address formatting and setup, as well as ensuring proper syntax for SMTP commands and changes to email client configurations.
Meanwhile, using Warmy.io for email warmup helps prevents these errors from occurring since it increases sender reputation and improves email deliverability.
Following this suggestion, you’ll fix SMTP Error 501 in no time and resume proper and productive emailing.
Sign up for a free trial today and experience the difference.
What does SMTP Error 501 indicate?
SMTP Error 501 indicates a syntax error in the email address or SMTP command line, meaning the server cannot process the request due to incorrect formatting.
Can incorrect email settings cause SMTP Error 501?
Yes, incorrect email settings, especially those related to the outgoing mail server (SMTP), can lead to SMTP Error 501. It’s important to ensure these settings are correctly configured.
How do I fix SMTP Error 501 in my email client?
To fix SMTP Error 501, verify the email address format, check your email client's SMTP settings for accuracy, and ensure you're using the correct syntax for any manual SMTP commands.
Does SMTP Error 501 mean my email was blocked?
Not necessarily. While SMTP Error 501 is related to syntax and formatting issues, it doesn't specifically indicate that your email was blocked. However, consistent errors can affect your sender reputation over time.
Are there tools to help prevent SMTP Error 501?
Yes, tools like Warmy.io offer email warm-up services and deliverability tests that can help improve your sender reputation and minimize the occurrence of SMTP errors, including Error 501.
Can SMTP Error 501 vary between email providers?
While the core issue behind SMTP Error 501 remains the same across providers, the way it's reported and the specific variations (e.g., 501 5.5.4, 501 5.1.7) can differ. Each provider may have unique guidelines for addressing the error.
Is SMTP Error 501 a permanent failure?
No, SMTP Error 501 is typically a temporary issue related to syntax errors that can be resolved by correcting the email format or SMTP command line syntax. It doesn’t indicate a permanent failure to send emails.
