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SMTP Email Error 535 5.7.3 - How to Resolve [SOLVED]

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    Email remains a cornerstone of modern communication, particularly in the business world where timely and reliable correspondence is crucial. At the heart of email technology is the Simple Mail Transfer Protocol (SMTP), which is responsible for sending emails from one server to another. When SMTP encounters issues, it can disrupt not only individual communications but also larger scale business operations.

    One common issue that can arise is SMTP Email Error 535 5.7.3, an error that typically indicates a problem with authentication—where the email server cannot verify the identity of the sender as legitimate. This error can prevent emails from being sent, leading to potential delays in communication and, in a broader sense, could impact customer relationships and business workflows. Understanding this error and knowing how to resolve it is essential for maintaining the efficiency and reliability of email communications.

    Deciphering SMTP email error 535 5.7.3

    SMTP Email Error 535 5.7.3 occurs whenever it is not possible to send an email due to failed sender authentication. While the actual wording may slightly differ from platform to platform, most error messages contain phrases such as “Authentication Credentials Invalid” or “Authentication Failed.” For example, it may be referred to as “Error 535 5.7.3 Authentication unsuccessful” in Microsoft Outlook or “Cannot send mail. The username or password for [SMTP server] is incorrect” in Apple Mail.

    The Email SMTP Error 535 5.7.3 indicates an issue with the authentication process. Authentication serves as a form of security measure that checks whether the user is allowed to send messages through the associated mail server. An error occurs whenever the server denies the verification credentials created by the local email client. There may be multiple reasons for the server rejection, including mistyped passwords and recent security changes or the platform not meeting the server’s default safety requirements.

    Identifying causes of SMTP email error 535 5.7.3

    Primarily, SMTP email 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.

    1. The first common cause is “using incorrect credentials”. This implies that the problem arises when one uses wrong login details, this includes password and username. Factors that prompt this mistake include a typing error, a recent password change that is not updated on the email client, or when the account user no longer has the old credentials that fail the server security level.
    2. The second cause of error 535 5.7.3 is misconfiguration. The misconfiguration of the email client settings may also prompt server authentication failure. When the email client is not configured to use the valid parameters, for example, the right authentication is needed by the SMTP server fails due to wrong server-port settings.
    3. A third cause is “Publisher security update”. An email provider might update their security settings or the kind of authentication protocol they offer. If the email client is not updated to the publish times, such as changing from less security to a more safe authentication method, it shows the 535 5.7.3 error.
    4. Another cause is the “Account Lockout” phenomenon. This means that the email host server might lock the accounts if it registers suspicious activities or when a breach has occurred. When the service provides files that there is no warning, the client receives the 535 5.7.3 error.
    5. Finally, the cause is “Software conflict” whereby the email client installed uses outdated software that no longer complies with the server’s settings or software with no updates will encounter the 535 5.7.3 error.

    Resolving SMTP email error 535 5.7.3: a step-by-step guide

    For Gmail Users

    1. 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.
    2. Enable Less 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.
    3. 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

    For Outlook Users

    1. Update Password. Ensure your password is correct, especially if you have recently changed it.
    2. 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

    For Yahoo Mail Users

    1. Password Verification. Confirm that the password and username are correctly entered in your email client.
    2. 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

    For Other Email Services

    • 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.

    Enhancing email efficiency with Warmy.io

    warmy dashboard

    While challenging to overcome, SMTP Email Error 535 5.7.3 can be prevented using Warmy.io. This tool can gradually increase the volume of all emails you send – a process known as warm-up – to gain a trusted reputation with email servers and avoid email bounds from being labeled as spam.

    In addition to that, Warmy.io helps you improve your email strategy overall – for example, through email deliverability and blacklist checks. Warmy.io has free SPF and DMARC record generators that can help you properly set up email authentication and add another layer of security to your emails. Just like importance, Warmy.io will help you maintain high email deliverability and avoid SMTP errors, keeping your communications uninterrupted and efficient.


    The importance of solving SMTP errors such as 535 5.7.3 cannot be stressed enough and may have a serious impact on efficient and reliable email. These errors can severely limit email-sending capabilities, damaging professional relationships, and the industry. Ensuring that your email settings are regularly managed is crucial to making your communication system functional and secure. Keeping your email configuration up to date and staying up to date with the most current security requirements will keep your email highly deliverable while limiting possible weaknesses. With the proper management and desire, you can maintain a reliable tool for communication.

    📜 Related article:


    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.

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