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SMTP Email Error 521 - How to Resolve [SOLVED]

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    Efficient email correspondence is one of the most important aspects of maintaining both personal and professional relationships and is widely used today. The prevalence of email communication makes errors such as SMTP, which may lead to disruption in using this service, especially hazardous.

    For example, the SMTP Email Error 521 is a potentially disruptive phenomenon because it directly influences the possibility of sending and receiving email. In short, SMTP Email Error 521 manifests when the recipient’s server is down or not willing to accept mail. This is particularly problematic from the sender’s perspective; in addition to not being able to deliver an email, it might signal other long-term issues. To understand the error’s appearance and implications, the sender is more likely to be able to solve the problem quickly and efficiently, and ensure the service’s functioning.

    Understanding SMTP email error 521

    A specific error message that indicates the SMTP server is not accepting connections is SMTP Email Error 521. This error usually indicates that the email server is not accepting any incoming emails, or it is offline. It is an important indicator that there can be serious problems with the configuration or availability of the email server.

    What SMTP Email Error 521 Indicates About an Email Server

    – Server Unavailability. Error 521 most frequently indicates that the server is not available at this time. This can be the result of maintenance, a server outage, or a permanent server shutdown.
    – Rejection of Emails. This error may also indicate that all incoming mail is being rejected by the server. The server administrator may have purposefully configured this, either in response to security issues or to control server load.

    Frequently Occurring Error Messages Related to Error 521

    When a user encounters SMTP Email Error 521, they may see a number of warnings indicating that the server is not accepting emails. Typical error messages consist of the following:

    • “521 5.2.1 : The mail server is not accepting connections.”
    • “521 5.3.0 Upstream server is not accepting connections.”
      “Delivery Status Notification (Failure)
    •  521: Host server is down.”

    The mail server of the recipient generates these messages, which are then returned to the sender’s email system. They act as alerts that the recipient server’s current state prevents the email from being delivered. It is essential to comprehend these messages in order to diagnose the problem and choose the best course of action for returning email functionality to normal.

    Resolving SMTP email error 521: causes and solutions

    1. Server Unavailability

    • Upgrading and maintenance. A server may become momentarily unavailable due to planned maintenance or upgrades. Error 521 may occur if the server is unable to receive inbound emails during these times.
    • Unexpected Downtime. The server may have unexpected downtime during which it is unable to accept emails due to hardware malfunctions, software crashes, or network problems.

    2. Configuration Issues

    • Deliberate Email Blocking. In order to reduce risks during an attack or as a security precaution against spam, administrators may set up servers to refuse all incoming emails.
    • Misconfiguration. The server may inadvertently reject inbound connections due to incorrect settings in its SMTP configuration. This can be the result of incorrect SMTP protocol configuration or firewall settings that prevent inbound email connections.

    3. Email Server Policies

    • Anti-Spam Rules. Strict anti-spam rules set up on some servers may incorrectly identify incoming emails as spam or malicious, which could result in their rejection.
    • IP Blacklisting. Emails received from a sending server may be refused if the receiving server has the sending server’s IP address on a blacklist.

    Impact on Email Delivery

    • Non-Delivery Reports (NDRs). Senders may experience communication disruptions and delays in the transfer of crucial information. They will receive a bounce-back message indicating that their email could not be sent.
    • Sender Reputation. A sender’s reputation may be impacted by recurrent SMTP problems, such as Error 521, which increases the likelihood that future servers may filter or flag their emails as spam.

    Step-by-step solutions to resolve SMTP email error 521

    For Gmail Users

    1. Check the Recipient’s Email Address and Status. Make sure the recipient’s email address is accurate and see if their email server is up and running right now. To verify the condition of the server, you can utilize programs such as MXToolbox.
    2. Examine Any Gmail Notificationsю Certain warnings or error messages from Gmail may reveal information about the root of the problem. Observe any guidelines or suggestions given.

    For Outlook Users

    1. Verify your account’s settings. Verify that all of your account’s settings—particularly the incoming and outgoing server settings—are configured appropriately. Make sure your authentication methods and server address are valid.
      Make use of the
    2. Microsoft Recovery and Support Assistant. One very useful tool that Microsoft provides for troubleshooting Outlook issues is for diagnosing and fixing Error 521.

    For Yahoo Users

    1. Verify Server Connectivity. As with other services, make sure there isn’t a server outage for the receiver. To find out the status of the server, use third-party software.
    2. Verify your email settings. Verify that your Yahoo Mail account’s SMTP settings are set correctly. The inability to send emails can be caused by incorrect SMTP settings.

    General Troubleshooting Steps

    1. Ping the Email Server. To verify connectivity, ping the recipient’s mail server using the Command Prompt or Terminal. To find out if the server is reachable, do a quick command like ping mail.example.com.
    2. Verify DNS Records. Make sure that the recipient’s domain’s MX records are successfully linked to a live mail server by using DNS lookup tools. Error 521 can frequently be caused by incorrect MX records.
    3. Examine the server logs. Examine the logs of the mail server if you can access it and see if there are any errors or warnings that might explain why the server is rejecting incoming emails.

    Advanced troubleshooting techniques for SMTP email error 521

    Network Diagnostics

    1. Network Analysis Tools. Utilize tools like Wireshark to capture and analyze network traffic. This can help identify if the SMTP traffic is being blocked or failing at any point in the network.
    2. Advanced Ping and Traceroute Tests. Perform advanced ping tests using options like packet size adjustment, or traceroute tests to map the route data takes to reach the destination server and identify where drops are occurring.
      • Command Example: ping -l 1500 mailserver.example.com (to test with larger packet size)

    Server Configuration and Logs

    1. SMTP Server Logs. Deep dive into SMTP server logs to look for error codes or messages that specifically relate to Error 521. This can often provide direct clues about the cause of the problem.
    2. Verify SMTP Server Configuration. Ensure that the configuration of your SMTP server is correct. This includes checking settings like port numbers, timeout settings, and whether the server is set to accept incoming connections.
      • Configuration File Location (Linux): /etc/postfix/main.cf for Postfix, /etc/exim.conf for Exim

    Security and Access Controls

    1. Firewall and Security Rules. Check the firewall settings on both the mail server and network devices to ensure that SMTP connections are not being inadvertently blocked.
      • Command to check firewall rules on Linux: iptables -L
    2. SELinux Policies. If you are running a server with SELinux enabled, make sure that the security policies are not preventing the SMTP service from operating correctly.
      • Command to check SELinux status: sestatus
      • Command to manage SELinux for SMTP: semanage port -a -t smtp_port_t -p tcp 25

    Email Server Health Checks

    1. Check for Blacklisting. Use DNSBL (DNS-based Blackhole List) checks to see if your server’s IP is blacklisted, which could be causing other servers to reject connections.
    2. SMTP Server Performance Monitoring. Regularly monitor the performance of your SMTP server using tools like Nagios or Zabbix, which can alert you to issues before they cause significant problems.

    Exploring additional variations of SMTP email errors 522 and 523

    Understanding SMTP email error 522

    SMTP email error 522, “Recipient has exceeded mailbox limit,” occurs when an email cannot be delivered because the recipient’s mailbox has reached its capacity and can no longer accept new messages. This typically results in a non-delivery report (NDR) or bounce-back message being sent to the sender.

    The error message might look like this:

    “Delivery has failed to these recipients or groups: [recipient’s email address]. The recipient’s mailbox is full and can’t accept messages now.”

    To resolve this issue, the recipient needs to free up space in their mailbox by deleting old emails or by increasing the mailbox size limit through their email service provider.

    Dealing with SMTP error 523

    SMTP email error 523, “Server limit exceeded. Message too large,” occurs when the email sent is too large to be processed by the recipient’s mail server. This is often due to attachments that exceed the server’s maximum permitted size for email messages.

    The sender typically receives a notification that the email could not be delivered, with a message such as

    “Message size exceeds fixed maximum message size.”

    To address this issue, the sender should reduce the size of the email by compressing attachments, removing unnecessary files, or splitting the content into multiple smaller emails. Alternatively, using an alternative method to share large files, such as cloud storage services, can prevent this error.

    Preventive measures and best practices for SMTP email error 521


    In order to avoid SMTP Email Error 521 or other such inconveniences, one should use Warmy.io to ensure smooth and reliable email delivery. As just mentioned, Warmy.io offers a free email deliverability test, that can help you discover and eliminate potential issues such as being classified as spam that can result in SMTP errors.

    What is more, Warmy.io has useful tools that may help you generate SPF and DMARC records for authentication of your email delivery and prevent them from being rejected by recipient servers. Keep using and updating this services to monitor and adjust your email settings and server configurations so that you can keep your email operations running day in, day out.


    This post has discussed SMTP Email Error 521, including its causes, effects, and thorough fixes for widely used email providers like Yahoo, Gmail, and Outlook. Advanced debugging procedures for deeper server issues have also been presented, and the significance of technologies like Warmy.io—which enhance email deliverability and assist in setting up vital SPF and DMARC records—has been emphasized.

    Proactively managing your email systems is essential to consistent and seamless email communication. Essential actions include routinely checking deliverability and authentication of emails, adjusting setups, and utilizing tools for this purpose. You may avoid common email errors like SMTP Email Error 521 and keep up productive communication lines by adopting these preventative steps.

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