When it comes to email, nothing is more essential than getting your message across, especially when money is to be made. Thus, emails need to be sent and received in the fastest, most efficient way possible. However, when an email deliverability test then shows that your emails are landing in spam, then this poses a problem.
Among your potential frustrations, SMTP Email Error 535 5.7.3 is one of the most common. This particular SMTP email error is usually related to authentication errors—meaning the system is not convinced who is trying to send what at the moment.
This means the email cannot be sent, causing problems with client relationships and stalling internal operations.
Understanding what this error code means and how you can fix the problem will ensure your email operations stay on the up and up.
While email functions over many general protocols, the vast majority of sending and receiving email across servers falls under the Simple Mail Transfer Protocol (SMTP).
SMTP Error 535 5.7.3 means your email wasn’t sent because the server can’t authenticate the sender.
While the error notification message differs from program to program, the general message is similar to “Invalid Authentication Credential” or “Authentication Failed.”
- For example, Microsoft Outlook says “Error 535 5.7.3 Authentication unsuccessful,”
- Apple Mail says, “Cannot send mail. The username or password for [SMTP server] is incorrect.”
Q: What is authentication?
A: Authentication is a form of security verification that guarantees a user has permission to send through the specific mail server. So when the email program on the host machine creates these necessary credentials for authentication and the server denies them, an error is generated.
🔖 Related Reading: Understanding and Fixing the SMTP 535 Error: Incorrect Authentication Data
Primarily, SMTP Error 535 5.7.3 occurs when there are issues with the authentication process of the server required to send emails.
Below are some possible causes that could lead to error 535 5.7.3.
Using invalid or outdated credentials
This implies that there is something wrong with the password and ID. Possible causes include:
- Some sort of typographical error
- A password change has not propagated yet through the email client
- The email user used to have access but does not have access anymore
- The old credentials do not comply with the new server security settings
Misconfiguration of server-port configurations
Another reason for server authentication errors is misconfiguration. For instance, what the email client configured doesn’t match what’s really valid.
Since the SMTP server needs proper authentication, this fails because the server-port configurations are wrong.
Publisher security update
This means something changed on the publisher/email service provider’s end in terms of security settings or the authentication it offers.
Should the email recipient fail to follow what’s been published—meaning, if it stays at a less secure authentication when the more secure authentication is now required—this is the response it receives for noncompliance.
Account lockout phenomenon
The email host server might lock the accounts if it registers suspicious activities or when a breach has occurred.
Software condlicts
The email client installed uses outdated software that no longer complies with the server’s required settings.
- Verify password and username. Verify that the username and password in your email client are current and accurate. Update your Gmail password in your email client as well, if you’ve changed it recently.
- Enableless secure apps (if necessary). You might need to enable “less secure apps” in your Gmail settings if you’re using an older email client. Nonetheless, using email clients that support OAuth is advised.
- Verify SMTP settings:
- SMTP Server: smtp.gmail.com
- Port: 587 for TLS/SSL
- Authentication required: Yes
- Username: Your full Gmail address
- Password: Your Gmail password
- Update password. Ensure your password is correct, especially if you have recently changed it.
- Check SMTP Settings:
- SMTP Server: smtp-mail.outlook.com
- Port: 587
- Encryption Method: STARTTLS
- Authentication Required: Yes
- Username: Your full Outlook email address
- Password: Your Outlook password
- Password verification. Confirm that the password and username are correctly entered in your email client.
- Adjust SMTP settings:
- SMTP Server: smtp.mail.yahoo.com
- Port: 587
- Encryption Method: STARTTLS
- Authentication Required: Yes
- Username: Your full Yahoo email address
- Password: Your Yahoo password
- Confirm authentication details. Double-check your username and password. Reset your password if unsure and update the new password in the email client.
- Correct SMTP settings. Refer to your email provider’s help documentation for the correct SMTP server settings, including server address, port, and encryption method.
- Contact support. If you continue to have issues, contact your email provider’s technical support for guidance on specific settings.
SMTP Email Error 535 5.7.3 workarounds may be challenging however, Warmy.io can help.
It’s true that SMTP Error 535 5.7.3 mostly covers authentication and configuration issues. But when it comes to email deliverability, there are other factors senders must keep an eye on.
With Warmy, all of these aspects are covered and taken care of with a wide array of tools and features.
AI-driven email warmup to prevent server rejections and spam flags
Warmy.io helps improve your sending reputation which is currently based on the performance and history of all emails you’ve ever sent. Essentially, Warmy helps you be seen as a legitimate sender by various email servers so your future emails will land in inboxes—and not the spam folder.
Warmy.io’s AI-powered email warmup helps your deliverability through:
- Gradually increasing email volume to prevent sudden spikes that could trigger mail server rejections.
- Simulating human-like interactions to ensure emails are opened, replied to, and marked as important.
- Monitoring email reputation to identify potential issues before they impact campaigns.
Free email deliverability test for diagnosing issues
Warmy.io’s Free Email Deliverability Test helps identify technical and reputation-based issues before they impact your campaigns. The test gives a comprehensive report of the percentage of your emails landing in the inbox, promotions, spam—and even the unreceived ones.
It also reveals if your domain or IP is included in any blacklists—yet another major factor that affects deliverability.
Q: What are blacklists and how do these impact deliverability?
A: Blacklists are databases that track IP addresses or domains known for sending spam or engaging in suspicious email behavior. Being blacklisted can lead to emails being blocked, sent to spam, or throttled.
🔖 Related Reading: Are Blacklists Killing Your Emails? A Deep Dive into How They Influence Email Providers
The test also checks your email authentication settings (SPF, DKIM, and DMARC) if they are properly authenticated.
Additional warmup preferences to maximize inbox placement
Warmup Preferences helps senders have full control over the warmup process from both sender and user levels.
Here’s what that looks like:
- Senders will be able to customize the warmup’s distribution across different providers.
- They can also choose between B2B or B2C customers for engagement patterns to tailor the behavior and insights to their business type.
Advanced seed list ensures strong inbox placement
Other traditional warmup lists can only simulate opening of emails, but Warmy’s advanced seed lists offer actual engagement.
What does this look like? Your emails are opened, scrolled through, clicked on, and replied to. If there are emails that land in spam, these are removed and marked as important to let the ESPs know you are credible.
It can also be integrated into any email client to improve your email performance testing. Warmy will supply genuine email addresses from Google, Outlook, and Yahoo that will act as seeds for testing and improving your email deliverability.
The need for troubleshooting SMTP Error 535 5.7.3 is essential.
After all, this error prevents you from being able to send emails—which jeopardizes customer relationships, fails to impress potential business connections, and puts one’s professional reputation at risk.
Therefore, troubleshooting this issue and understanding proper configurations of the email settings is necessary.
However, configurations are just one part of the email deliverability landscape. There are many other factors that impact your deliverability and simply focusing on resolving SMTP Error 535 5.7.3 will not automatically get rid of your deliverability issues.
Thankfully, Warmy provides tools and strategies that can cover deliverability as a whole.
Book a demo with an expert today and see how it can transform your email deliverability and business.
📜 Related articles:
What is SMTP Email Error 535 5.7.3?
When an email client provides the SMTP server with erroneous or unaccepted login credentials, the SMTP email error 535 5.7.3 arises, prohibiting email from being sent.
What is the reason of SMTP Email Error 535 5.7.3?
This error usually occurs when the SMTP server's authentication procedures are incorrectly setup, the username or password is entered incorrectly, or the account security settings are altered.
How can I resolve my email client's SMTP Email Error 535 5.7.3?
Make sure the right username and password are entered in your email client's settings to fix this error. Additionally, confirm that the authentication criteria of your email provider haven't been altered. It can also be beneficial to change your password in your email client and reset it.
Does SMTP Email Error 535 5.7.3 get fixed by changing the SMTP port?
This problem usually cannot be fixed by changing the SMTP port on its own unless there is a specific port misconfiguration involved. Since authentication is mostly the cause of this problem, pay close attention to your security settings and appropriate credentials.
Can I repair the problem by updating my email client software?
It's true that occasionally underlying problems that could be producing login difficulties, like Error 535 5.7.3, can be fixed by updating your email software. Ensure that the most recent security and functionality updates are installed in your email client.
How can I tell if the security settings on my email server are the reason for SMTP Error 535 5.7.3?
To find out whether there have been any changes to the security or authentication policies that might be affecting your ability to send emails, see the documentation or assistance provided by your email service provider.
