{"id":3869,"date":"2024-04-18T11:09:39","date_gmt":"2024-04-18T11:09:39","guid":{"rendered":"https:\/\/www.warmy.io\/blog\/how-to-fix-smtp-email-error-521-solved\/"},"modified":"2026-04-05T11:44:41","modified_gmt":"2026-04-05T11:44:41","slug":"how-to-fix-smtp-email-error-521-solved","status":"publish","type":"post","link":"https:\/\/www.warmy.io\/blog\/how-to-fix-smtp-email-error-521-solved\/","title":{"rendered":"SMTP Error 521: Host Does Not Accept Mail for This Domain \u2014 Causes and Fixes"},"content":{"rendered":"<p><span>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\u2002emergence of social media networks. However, the prevalence of email communication makes errors such as <\/span><a href=\"https:\/\/www.warmy.io\/blog\/what-is-smtp-and-how-does-the-smtp-server-work\/\" target=\"_blank\" rel=\"noopener noreferrer\"><span>Simple Mail Transfer Protocol (SMTP)<\/span><\/a><span> especially hazardous\u2014because they may lead to disruption in using this service,\u00a0<\/span><\/p>\n<p><span>Some errors might be more detrimental than others. The SMTP Email Error 521, for\u2002example, could literally prevent you from sending and receiving emails. <\/span><b>Simply put, SMTP Email Error 521 occurs when the recipient\u2019s server is either down or unwilling to accept the mail.<\/b><\/p>\n<p><span>This can be particularly frustrating and troubling from the sender\u2019s 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\u2019re much more likely to\u2002fix the issue quickly\u2014and stop it from happening again.<\/span><\/p>\n<p><iframe width=\"640\" height=\"360\" src=\"https:\/\/www.youtube.com\/embed\/2pZ3Bbp0g84\" frameborder=\"0\" allowfullscreen allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\"><\/iframe><\/p>\n<h2>Understanding SMTP email error 521<br \/>\n<\/h2>\n<p><span>SMTP Email Error\u2002521 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\u2002sign that there may be a significant issue on an email server\u2019s configuration or availability.<\/span><\/p>\n<h3>What does SMTP Email Error 521 indicate about an email server?<br \/>\n<\/h3>\n<ul>\n<li><b>Server unavailability. <\/b><span>SMTP Email Error 521 usually indicates that the server is not available at this time.<\/span><span> This can be the result of maintenance, a server outage, or a permanent server shutdown.<\/span><\/li>\n<li><b>Rejection of emails. <\/b><span>This error may also indicate that all incoming emails are being rejected by the server. <\/span><span>The server administrator may have purposefully configured this, either in response to security issues or to control server load.<\/span><\/li>\n<\/ul>\n<h3>Frequently occurring error messages related to SMTP Email Error 521<br \/>\n<\/h3>\n<p><span>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:<\/span><\/p>\n<ul>\n<li><b>521 5.2.1:<\/b><span> The mail server is not accepting connections.<\/span><\/li>\n<li><b>521 5.3.0: <\/b><span>Upstream server is not accepting connections. Delivery Status Notification (Failure)<\/span><\/li>\n<li><b>521:<\/b><span> Host server is down.<\/span><\/li>\n<\/ul>\n<p><span>The mail server of the recipient generates these messages, which are then returned to the sender\u2019s email system. They act as alerts that the recipient server\u2019s 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.<\/span><\/p>\n<h3>How does SMTP Email Error 521 impact email delivery?<br \/>\n<\/h3>\n<ul>\n<li><b>Non-Delivery Reports (NDRs).<\/b><span> 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.<\/span><\/li>\n<li><b>Sender reputation.<\/b><span> A sender\u2019s 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.<\/span><\/li>\n<\/ul>\n<h2>Resolving SMTP Email Error 521: causes and solutions<\/p>\n<\/h2>\n<h3>1. Server unavailability<\/p>\n<\/h3>\n<ul>\n<li><b>Upgrading and maintenance.<\/b><span> A server may become momentarily unavailable due to planned maintenance or upgrades<\/span><span>. Error 521 may also occur if the server is unable to receive inbound emails during these times.<\/span><\/li>\n<li><b>Unexpected downtime.<\/b><span> The server may experience unexpected downtime. When this happens, it is unable to accept emails due to hardware malfunctions, software crashes, or network problems.<\/span><\/li>\n<\/ul>\n<h3>2. Configuration issues<\/p>\n<\/h3>\n<ul>\n<li><b>Deliberate email blocking.<\/b> <span>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.<\/span><\/li>\n<li><b>Misconfiguration<\/b><span>. The server may inadvertently reject inbound connections due to incorrect settings in its SMTP configuration. <\/span><span>This can be the result of incorrect SMTP protocol configuration or firewall settings that prevent inbound email connections.<\/span><\/li>\n<\/ul>\n<h3>3. Email server policies<\/p>\n<\/h3>\n<ul>\n<li><b>Anti-spam rules. <\/b><span>Strict anti-spam rules set up on some servers may incorrectly identify incoming emails as spam or <\/span><span>malicious\u2014which could result in their rejection.<\/span><\/li>\n<li><a href=\"https:\/\/www.warmy.io\/blog\/how-to-check-ip-reputation-ip-blacklists\/\" target=\"_blank\" rel=\"noopener noreferrer\"><b>IP blacklisting<\/b><b>.<\/b><\/a><span> Emails received from a sending server may be refused if the receiving server has the sending server\u2019s IP address on a blacklist.<\/span><\/li>\n<\/ul>\n<h2>Step-by-step solutions to resolve SMTP Email Error 521<\/h2>\n<h3>For Gmail users<\/h3>\n<ol>\n<li><b>Check the recipient\u2019s email address and status. <\/b><span>Make sure the recipient\u2019s email address is accurate and verify if their email server is up and running right now. <\/span><span>To verify the condition of the server, you can utilize programs such as MXToolbox.<\/span><\/li>\n<li><b>Examine your Gmail notifications.<\/b><span> Certain warnings or error messages from Gmail may reveal information about the root of the problem. Observe any guidelines or suggestions given.<\/span><\/li>\n<\/ol>\n<h3>For Outlook users<br \/>\n<\/h3>\n<ol>\n<li><b>Verify your account\u2019s settings. <\/b><span>Verify that all of your account\u2019s settings\u2014particularly the incoming and outgoing server settings\u2014are configured correctly. Make sure your authentication methods and server address are also valid.<\/span><\/li>\n<li><b>Make use of the Microsoft Recovery and Support Assistant.<\/b><span> This is one very useful tool that Microsoft provides for troubleshooting Outlook issues for diagnosing and fixing Error 521.<\/span><\/li>\n<\/ol>\n<h3>For Yahoo! users<\/h3>\n<ol>\n<li><b>Verify server connectivity.<\/b><span> As with other services, make sure there isn\u2019t a server outage for the receiver. To find out the status of the server, use third-party software.<\/span><\/li>\n<li><b>Verify your email settings.<\/b><span> Verify that your Yahoo! Mail account\u2019s SMTP settings are set correctly. The inability to send emails can be caused by incorrect SMTP settings.<\/span><\/li>\n<\/ol>\n<h3>General troubleshooting steps<\/h3>\n<ol>\n<li><b>Ping the email server.<\/b> <span>To verify connectivity, ping the recipient\u2019s 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.<\/span><\/li>\n<li><b>Verify DNS records.<\/b><span> Make sure that the recipient\u2019s domain\u2019s 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.<\/span><\/li>\n<li><b>Examine the server logs.<\/b><span> Examine the logs of the mail server. <\/span><span>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.<\/span><\/li>\n<\/ol>\n<h2>Advanced troubleshooting techniques for SMTP Email Error 521<br \/>\n<\/h2>\n<h3>Network diagnostics<br \/>\n<\/h3>\n<ol>\n<li><strong>Network Analysis Tools.<\/strong> <span>Utilize tools like <\/span><a href=\"https:\/\/www.wireshark.org\/\" rel=\"noopener\" target=\"_blank\" rel=\"noopener noreferrer\"><span>Wireshark<\/span><\/a><span> 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.<\/span><\/li>\n<li><b>Advanced ping and traceroute tests. <\/b><span>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.<\/span>\n<ul>\n<li>Command Example: <code>ping -l 1500 mailserver.example.com<\/code> (to test with larger packet size)<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<h3>Server configuration and logs<br \/>\n<\/h3>\n<ol>\n<li><strong>SMTP server logs.<\/strong>\u00a0Deep 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.<\/li>\n<li><b>Verify SMTP server configuration<\/b><span>. This could also be related\u2002to configuration of SMTP server. This involves checking things like port numbers, timeout\u2002options and whether the server is indeed set up to listen for incoming connections.<\/span>\n<ul>\n<li>Configuration File Location (Linux): <code>\/etc\/postfix\/main.cf<\/code> for Postfix, <code>\/etc\/exim.conf<\/code> for Exim<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<h3>Security and access controls<br \/>\n<\/h3>\n<ol>\n<li><b>Firewall and security rules.<\/b><span> Verify whether the firewall rules on the mail server as well as any other network devices are being used to block SMTP connections.<\/span>\n<ul>\n<li>Command to check firewall rules on Linux: <code>iptables -L<\/code><\/li>\n<\/ul>\n<\/li>\n<li><b>SELinux policies. <\/b><span>If you are running a server with SELinux enabled, ensure that the security policies don\u2019t restrict the SMTP service from functioning properly.<\/span>\n<ul>\n<li>Command to check SELinux status: <code>sestatus<\/code><\/li>\n<li>Command to manage SELinux for SMTP: <code>semanage port -a -t smtp_port_t -p tcp 25<\/code><\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<p>Beyond per-incident fixes, complement SMTP hardening with <a href=\"https:\/\/www.wiz.io\/academy\/cloud-security\/exposure-management-in-cybersecurity\" rel=\"noopener\" target=\"_blank\" rel=\"noopener noreferrer\">exposure management best practices<\/a> to continuously surface internet-facing assets, prioritize exploitable misconfigurations, and enforce least privilege across cloud and on\u2011prem environments\u2014reducing downtime risks and preventing protective controls from inadvertently blocking legitimate mail flow.<\/p>\n<h3>Email server health checks<\/h3>\n<ol>\n<li><b>Check for blacklisting.<\/b><span> Use DNSBL (<\/span><a href=\"https:\/\/www.warmy.io\/blog\/dnsbl-blacklist-everything-you-need-to-know\/\" target=\"_blank\" rel=\"noopener noreferrer\"><span>DNS-based Blackhole List<\/span><\/a><span>) checks to see if your server\u2019s IP is blacklisted,<\/span><span> which could be causing other servers to reject connections.<\/span>\n<ul>\n<li><span>Related reading: <\/span><a href=\"https:\/\/www.warmy.io\/blog\/email-domain-ip-blacklist-removal-steps-to-delist-your-ip\/\" target=\"_blank\" rel=\"noopener noreferrer\"><span>Email Domain &amp; IP Blacklist Removal: 5 Steps to Delist<\/span><\/a><\/li>\n<\/ul>\n<\/li>\n<li><b>SMTP server performance monitoring.<\/b><span> 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.<\/span><\/li>\n<\/ol>\n<h2>Exploring additional variations of SMTP Email Error 522 and SMTP Email Error 523<br \/>\n<\/h2>\n<h3>Understanding SMTP Email Error 522<br \/>\n<\/h3>\n<p><span>SMTP Email Error 522, \u201cRecipient has exceeded mailbox limit.\u201d This SMTP email error simply means that your email can\u2019t be delivered because the recipient mailbox is full\u2002and 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.<\/span><\/p>\n<p><span>The error message might look like this: <\/span><\/p>\n<blockquote>\n<p>\u201cDelivery has failed to these recipients or groups: [recipient\u2019s email address]. The recipient\u2019s mailbox is full and can\u2019t accept messages now.\u201d<\/p>\n<\/blockquote>\n<p><span>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.<\/span><\/p>\n<h3>Dealing with SMTP Email Error 523<\/h3>\n<p><span>SMTP Email Error 523, \u201cServer limit exceeded. Message too large,\u201d occurs when <\/span><b>the email sent is too large to be processed by the recipient\u2019s mail server. <\/b><span>This is often due to attachments that exceed the server\u2019s <\/span><span>maximum allowed size for email messages.<\/span><\/p>\n<p><span>The sender will usually receive a notification that the email could not be delivered, with a message such as<\/span><\/p>\n<blockquote>\n<p>\u201cMessage size exceeds fixed maximum message size.\u201d<\/p>\n<\/blockquote>\n<p><span>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.<\/span><\/p>\n<h2>Use Warmy.io for proactive measures against SMTP Email Error 521 and others<br \/>\n<\/h2>\n<p><img loading=\"lazy\" decoding=\"async\" alt=\"dashboard\" height=\"600\" src=\"https:\/\/warmy-blog-wordpress-bucket.s3.amazonaws.com\/wp-content\/uploads\/2024\/03\/11093816\/dashboard-1024x768.png\" width=\"800\" title=\"\"><\/p>\n<p><span>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.\u00a0<\/span><\/p>\n<p><span>To avoid SMTP Email Error 521 or other similar errors, email senders may want to look into using <\/span><a href=\"https:\/\/warmy.io\" rel=\"noopener\" target=\"_blank\" rel=\"noopener noreferrer\"><span>Warmy.io<\/span><\/a><span> to ensure reliable email deliverability. Warmy.io also offers a <\/span><a href=\"https:\/\/www.warmy.io\/free-tools\/email-deliverability-test\" target=\"_blank\" rel=\"noopener noreferrer\"><span>free email deliverability test,<\/span><\/a><span> that can help discover and eliminate potential issues that can result in SMTP error (such as being classified as spam, for example).<\/span><\/p>\n<p><span>Warmy.io has additional free tools that can help you generate <\/span><a href=\"https:\/\/www.warmy.io\/free-tools\/spf-generator\" target=\"_blank\" rel=\"noopener noreferrer\"><span>SPF<\/span><\/a><span> and <\/span><a href=\"https:\/\/www.warmy.io\/free-tools\/dmarc-generator\" target=\"_blank\" rel=\"noopener noreferrer\"><span>DMARC<\/span><\/a><span> 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\u2014day in and day out.<\/span><\/p>\n<p><span>\ud83d\udcdc Related article:<\/span><\/p>\n<ul>\n<li><a href=\"https:\/\/www.warmy.io\/blog\/smtp-email-error-512-how-to-resolve-solved\/\" target=\"_blank\" rel=\"noopener noreferrer\"><span>SMTP Email Error 512 \u2013 How to Resolve [SOLVED]<\/span><\/a><\/li>\n<li><a href=\"https:\/\/www.warmy.io\/blog\/how-to-fix-smtp-email-error-530-solved\/\" target=\"_blank\" rel=\"noopener noreferrer\"><span>SMTP Email Error 530 \u2013 How to Resolve [SOLVED]<\/span><\/a><\/li>\n<li><a href=\"https:\/\/www.warmy.io\/blog\/how-to-fix-smtp-email-error-502-solved\/\" target=\"_blank\" rel=\"noopener noreferrer\"><span>SMTP Email Error 502 \u2013 How to Resolve [SOLVED]<\/span><\/a><\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>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\u2002emergence of social media networks. However, the prevalence of email communication makes errors such as Simple Mail Transfer Protocol (SMTP) especially hazardous\u2014because they may lead to disruption in using [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":4648,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[104],"tags":[],"class_list":["post-3869","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-email-deliverability"],"acf":[],"lang":"en","translations":{"en":3869},"pll_sync_post":[],"_links":{"self":[{"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/posts\/3869","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/comments?post=3869"}],"version-history":[{"count":1,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/posts\/3869\/revisions"}],"predecessor-version":[{"id":5743,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/posts\/3869\/revisions\/5743"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/media\/4648"}],"wp:attachment":[{"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/media?parent=3869"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/categories?post=3869"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/tags?post=3869"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}