Efficient communication via email is one of the main aspects of maintaining both personal and professional relationships. That explains why email continues to flourish today, despite the emergence of social media networks. However, the prevalence of email communication makes errors such as Simple Mail Transfer Protocol (SMTP) especially hazardous—because they may lead to disruption in using this service,
Some errors might be more detrimental than others. The SMTP Email Error 521, for example, could literally prevent you from sending and receiving emails. Simply put, SMTP Email Error 521 occurs when the recipient’s server is either down or unwilling to accept the mail.
This can be particularly frustrating and troubling from the sender’s perspective. This is not about not being able to send that particular email, it can mean other long-term issues. But, if the sender can understand and diagnose, then they’re much more likely to fix the issue quickly—and stop it from happening again.
SMTP Email Error 521 is a type of error message which means the SMTP server is currently down, or is not accepting any incoming emails. This is a critical sign that there may be a significant issue on an email server’s configuration or availability.
What does SMTP Email Error 521 indicate about an email server?
- Server unavailability. SMTP Email Error 521 usually 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 emails are 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 SMTP Email 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 is preventing the email from being delivered. It is essential to understand these messages in order to diagnose the problem and choose the best course of action for improving email deliverability.
How does SMTP Email Error 521 impact 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 recurring SMTP problems (such as Error 521) which increases the likelihood that future servers may filter or flag their emails as spam.
- Upgrading and maintenance. A server may become momentarily unavailable due to planned maintenance or upgrades. Error 521 may also occur if the server is unable to receive inbound emails during these times.
- Unexpected downtime. The server may experience unexpected downtime. When this happens, it is unable to accept emails due to hardware malfunctions, software crashes, or network problems.
- Deliberate email blocking. This happens when administrators set up servers to refuse all incoming emails in order to reduce risks during an attack or as a security precaution against spam.
- 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.
- 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.
- Check the recipient’s email address and status. Make sure the recipient’s email address is accurate and verify if their email server is up and running right now. To verify the condition of the server, you can utilize programs such as MXToolbox.
- Examine your Gmail notifications. Certain warnings or error messages from Gmail may reveal information about the root of the problem. Observe any guidelines or suggestions given.
- Verify your account’s settings. Verify that all of your account’s settings—particularly the incoming and outgoing server settings—are configured correctly. Make sure your authentication methods and server address are also valid.
- Make use of the Microsoft Recovery and Support Assistant. This is one very useful tool that Microsoft provides for troubleshooting Outlook issues for diagnosing and fixing Error 521.
- 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.
- 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.
- 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, you can do a quick command like ping mail.example.com.
- 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.
- Examine the server logs. Examine the logs of the mail server. Check if you can access it and see if there are any errors or warnings that might explain why the server is rejecting incoming emails.
- 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.
- 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)
- Command Example:
- 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.
- Verify SMTP server configuration. This could also be related to configuration of SMTP server. This involves checking things like port numbers, timeout options and whether the server is indeed set up to listen for incoming connections.
- Configuration File Location (Linux):
/etc/postfix/main.cffor Postfix,/etc/exim.conffor Exim
- Configuration File Location (Linux):
- Firewall and security rules. Verify whether the firewall rules on the mail server as well as any other network devices are being used to block SMTP connections.
- Command to check firewall rules on Linux:
iptables -L
- Command to check firewall rules on Linux:
- SELinux policies. If you are running a server with SELinux enabled, ensure that the security policies don’t restrict the SMTP service from functioning properly.
- Command to check SELinux status:
sestatus - Command to manage SELinux for SMTP:
semanage port -a -t smtp_port_t -p tcp 25
- Command to check SELinux status:
- 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.
- Related reading: Email Domain & IP Blacklist Removal: 5 Steps to Delist
- 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 a larger problem.
SMTP Email Error 522, “Recipient has exceeded mailbox limit.” This SMTP email error simply means that your email can’t be delivered because the recipient mailbox is full and cannot accept emails anymore. At least for the meantime. Most often, this results in an NDR (non-delivery report) 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.
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 allowed size for email messages.
The sender will usually receive 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. Using an alternative method to share large files, such as cloud storage services, can also prevent this error.
Proactively managing your email systems is essential to ensure consistent and seamless email communication. Necessary actions include routinely checking deliverability and authentication of emails, adjusting setups, and utilizing different tools for this purpose.
To avoid SMTP Email Error 521 or other similar errors, email senders may want to look into using Warmy.io to ensure reliable email deliverability. Warmy.io also offers a free email deliverability test, that can help discover and eliminate potential issues that can result in SMTP error (such as being classified as spam, for example).
Warmy.io has additional free tools that can help you generate SPF and DMARC records for authentication of your email delivery to prevent them from being rejected by recipient servers. Keep using and updating these services to monitor and adjust your email settings and server configurations so you can keep your email operations running smoothly—day in and day out.
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