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SMTP Email Error 504 5.5.2 - How to Resolve [SOLVED]

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    Simple Mail Transfer Protocol is the standard email protocol, and it is used to send email across the internet. SMTP is responsible for the email sending process and allows one email server to transmit an email to another email server. With such a crucial role, any error encountered in SMTP will affect communication to a large extent.

    Thus, delays in sending and delivery of messages, communication breakdown, lost correspondence, and possible disruption of business operations along with other personal communications will be inevitable. It will be necessary to address SMTP errors as quickly as they emerge because it can be impossible to send emails.

    Understanding SMTP email error 504 5.5.2

    SMTP Email Error 504 5.5.2 is a server-side error from the email transmission protocol. This error occurs when the SMTP command syntax is incorrect. While there may be other variations of the error warning as the user encounters the same error, the common warning that accompanies SMTP Email Error 504 5.5.2 is,

    504 5.5.2 <command>: Syntax error

    This recognition alert typically follows an SMTP command error during the emailing process, indicating that the command was incorrect or misformatted. By causing this error, there may be significant disruption in email service. For example, the user’s ability to send out simultaneous emails could be hampered.

    Common triggers for SMTP email error 504 5.5.2

    Incorrect Command Syntax

    One of the most frequent causes of this problem is an SMTP command that has the wrong syntax. Typing mistakes, missing parameters, or improper command string spacing could be the cause of this. There is a specific syntax that every command delivered to the SMTP server must follow; any departure may result in an error.

    Unsupported Commands

    A 504 error may occur if the SMTP server does not acknowledge or support a command sent by the email client. This frequently occurs when email clients that are out of date are used, or when the client’s and the server’s capabilities are not compatible.

    Configuration Errors

    The email client’s incorrectly configured SMTP settings may also result in this error. This includes user-specified wrong server addresses, ports, or security protocols, as well as default settings that don’t meet the needs of the server.

    Firewall or Security Settings

    SMTP commands may occasionally be blocked or have their syntax changed by network security technologies like firewalls or antivirus software. The server can interpret this interference as a syntactic problem.

    Server Issues

    Occasionally, the server may be the source of the issue rather than the client. Unexpected behavior, such as misinterpreting appropriately structured commands, might result from server-side problems including bugs, maintenance activities, or overloads.

    Detailed solutions for resolving SMTP email error 504 5.5.2 across various email clients

    For Gmail Users

    1. Check SMTP Settings. Ensure that your SMTP settings are correctly configured:

      • SMTP server: smtp.gmail.com
      • Port: 465 (SSL) or 587 (TLS)
      • Username: Your full Gmail address
      • Password: Your Gmail password or an App password if two-factor authentication is enabled.
    2. Use Correct Syntax. Double-check that all SMTP commands sent via any third-party email applications are correctly formatted according to the SMTP standards.

    3. Enable Less Secure Apps. If using less modern software or apps, you may need to enable less secure apps within Gmail settings to allow SMTP communication.

    For Outlook Users

    1. Verify Email Settings. Make sure your account settings in Outlook match the following:

      • Outgoing mail server (SMTP): smtp-mail.outlook.com
      • Port: 587
      • Encryption method: STARTTLS
    2. Update Outlook. Ensure that your Outlook application is up to date, as updates often include fixes for common bugs that could be causing the SMTP error.

    3. Re-enter Credentials. Sometimes, simply re-entering your password can resolve syntax errors that may occur if the password was changed or updated recently.

    For Yahoo Mail Users

    1. SMTP Configuration Check: Confirm that your SMTP settings are as follows:

      • SMTP server: smtp.mail.yahoo.com
      • Port: 465 (SSL) or 587 (TLS)
      • Username: Your full Yahoo email address
      • Password: Your Yahoo password.
    2. Authenticate Correctly. Verify that your email client is utilizing the appropriate authentication mechanism mandated by Yahoo. This may entail modifying the authentication configuration to leverage OAuth.

    3. Examine the Security Settings. You can prevent unknown apps from sending emails by using Yahoo’s security settings. You might have to grant access using the Yahoo Account Security settings.

    Enhancing email deliverability with Warmy.io

    warmy email deliverability

    Warmy.io is a convenient tool to improve email deliverability. For individuals and businesses frequently encountering SMTP error 504 5.5.2, this tool can be invaluable. Two main functions of Warmy.io that can improve email deliverability and reduce SMTP errors, as well as protecting against future occurrences, are warming up email addresses and provision of various deliverability tools. Warmy.io enables the user with several functions to improve email deliverability and minimize the risk of SMTP errors.

     ✅Warming up email addresses. Email warming is a process of progressively increasing the volume of emails one sends from a particular email address to establish a reliable reputation with Internet Service Providers . This function is critical for a new email account or a domain as high volumes of email trigger red flags for spam systems. Warmy.io automates this process by sending an increasing number of emails in a controlled manner. This allows not only to establish the user’s reputation but also to curb the rate of SMTP errors caused by spam filters and one’s poor reputation.

    ✅ Checking email deliverability. Warmy.io offers a free email deliverability test. Users can see how likely their email will be delivered to the recipient inbox by screening against various factors contributing to deliverability. By adjusting one’s approaches to email after learning of certain issues from the error report, one can minimize the risk of SMTP errors triggered by deliverability.

    Inspecting blacklists. Being blacklisted inhibits email deliverability and spurs numerous SMTP errors. Warmy.io offers users the ability to ascertain whether their email address is blacklisted by using email deliverability test.

    domain blacklist

    ✅ Setting up email authentication. Email authentication is essential to verify that the email sent from a domain actually originates from the purported sender . This critical measure is pivotal in identifying and curtailing impersonation and phishing attempts. Warmy.io allows free SPF generators and DMARC record generators to facilitate the process. Proper DNS implementation eliminates the risk of one’s email being flagged as spam or phishing.


    In this article, we have thoroughly considered the issue of solving the SMTP Email Error 504 5.5.2, we learned what steps should be taken to eliminate the problem on the example of the Gmail, Outlook and Yahoo mail email platforms, understood the importance of correctly configured SMTP, email client updates and the formatting of the command executed.

    In the course of the work, we have familiarized ourselves with Warmy.io, a tool that automatically warms up the address, checks the deliverability, verifies the existence of a site in blacklists and draws up the SPF and DMARC records. All of these tools help to avoid SMTP errors and make email communication reliable.

    The work done is important because the correct sending of emails is necessary for maintaining proper communication. No one will wait for an email for several hours, and network security is paramount. This article can help users to ensure a seamless flow of emails.

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