{"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":"2026-07-07T09:25:09","modified_gmt":"2026-07-07T09:25:09","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 &#8211; Causes and Fixes"},"content":{"rendered":"\n<p>Email remains central to both personal and professional communication, which is exactly why protocol-level failures like <a href=\"https:\/\/www.warmy.io\/blog\/what-is-smtp-and-how-does-the-smtp-server-work\/\" target=\"_blank\" rel=\"noopener noreferrer\">Simple Mail Transfer Protocol (SMTP)<\/a> errors are so disruptive &#8211; they stop that communication cold.<\/p>\n\n\n\n<p>SMTP Error 521 means the recipient\u2019s mail server is refusing to accept any incoming mail at all &#8211; the connection opens, then immediately closes with a 521 reply, whether because the host is down, offline, or deliberately configured to reject mail outright. It\u2019s one of the more disruptive SMTP errors precisely because it can block sending and receiving entirely, not just a single message.<\/p>\n\n\n\n<p>This code is formally defined in <a href=\"https:\/\/datatracker.ietf.org\/doc\/html\/rfc7504\" rel=\"noopener\" target=\"_blank\" rel=\"noopener noreferrer\">RFC 7504<\/a>, the IETF specification that reserves 521 for hosts that never accept mail under any circumstances &#8211; as distinct from servers that only reject specific messages or senders.<\/p>\n\n\n\n<p>Unresolved, this is rarely about one missed email. A recurring 521 usually points to a longer-term server, DNS, or configuration issue on the recipient\u2019s side, and continuing to retry against a host that returns it wastes time. Diagnosing the exact cause is the fastest way to fix it and stop it from recurring.<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p>\ud83d\udca1 Pro Tip: Before assuming the fault is on your end, run telnet mailserver.example.com 25 (or use an online SMTP tester) to see the exact reply the recipient\u2019s host returns. If a 521 shows up immediately at connection, the recipient\u2019s server is the one refusing mail &#8211; not your setup.<\/p>\n<\/blockquote>\n\n\n\n<h2 class=\"wp-block-heading\">Understanding SMTP error 521<\/h2>\n\n\n\n<p>SMTP 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.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">What does SMTP Error 521 indicate about an email server?<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><b>Server unavailability. <\/b>SMTP 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.<\/li>\n\n\n\n<li><b>Rejection of emails. <\/b>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.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Frequently occurring error messages related to SMTP Email Error 521<\/h3>\n\n\n\n<p>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:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><b>521 5.2.1:<\/b> The mail server is not accepting connections.<\/li>\n\n\n\n<li><b>521 5.3.0: <\/b>Upstream server is not accepting connections. Delivery Status Notification (Failure)<\/li>\n\n\n\n<li><b>521:<\/b> Host server is down.<\/li>\n<\/ul>\n\n\n\n<p>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. Don\u2019t confuse Error 521 with other codes that show up in the same bounce logs: <a href=\"https:\/\/www.warmy.io\/blog\/how-to-fix-smtp-email-error-501-solved\/\" target=\"_blank\" rel=\"noopener noreferrer\">SMTP Email Error 501<\/a> points to a syntax problem in the address or command, while <a href=\"https:\/\/www.warmy.io\/blog\/how-to-fix-smtp-email-error-502-solved\/\" target=\"_blank\" rel=\"noopener noreferrer\">SMTP Email Error 502<\/a> means the server received a command it doesn\u2019t support &#8211; neither means the recipient host is refusing mail outright, the way 521 does.<\/p>\n\n\n\n<p>For a complete breakdown of every SMTP status code &#8211; including the 4xx temporary failures not covered here &#8211; see Warmy\u2019s <a href=\"https:\/\/www.warmy.io\/blog\/smtp-error-codes-and-messages\/\" target=\"_blank\" rel=\"noopener noreferrer\">SMTP Error Codes: Complete List &amp; What They Mean<\/a>.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">How does SMTP Error 521 impact email delivery?<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><b>Non-Delivery Reports (NDRs).<\/b> 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.<\/li>\n\n\n\n<li><b>Sender reputation.<\/b> 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.<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">Resolving SMTP Error 521: causes and solutions<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">1. Server unavailability<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><b>Upgrading and maintenance.<\/b> 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.<\/li>\n\n\n\n<li><b>Unexpected downtime.<\/b> The server may experience unexpected downtime. When this happens, it is unable to accept emails due to hardware malfunctions, software crashes, or network problems.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">2. Configuration issues<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><b>Deliberate email blocking.<\/b> 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.<\/li>\n\n\n\n<li><b>Misconfiguration<\/b>. 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. If your server responds by demanding valid login credentials instead of refusing the connection outright, that\u2019s <a href=\"https:\/\/www.warmy.io\/blog\/how-to-fix-smtp-email-error-530-solved\/\" target=\"_blank\" rel=\"noopener noreferrer\">SMTP Email Error 530<\/a> (Authentication Required), not 521.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">3. Email server policies<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><b>Anti-spam rules. <\/b>Strict anti-spam rules set up on some servers may incorrectly identify incoming emails as spam or malicious-which could result in their rejection.<\/li>\n\n\n\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> Emails received from a sending server may be refused if the receiving server has the sending server\u2019s IP address on a blacklist.<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">Step-by-step solutions to resolve SMTP Error 521<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">For Gmail users<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><b>Check the recipient\u2019s email address and status. <\/b>Make sure the recipient\u2019s 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.<\/li>\n\n\n\n<li><b>Examine your Gmail notifications.<\/b> Certain warnings or error messages from Gmail may reveal information about the root of the problem. Observe any guidelines or suggestions given.<\/li>\n<\/ol>\n\n\n\n<h3 class=\"wp-block-heading\">For Outlook users<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><b>Verify your account\u2019s settings. <\/b>Verify that all of your account\u2019s settings &#8211; particularly the incoming and outgoing server settings &#8211; are configured correctly. Make sure your authentication methods and server address are also valid.<\/li>\n\n\n\n<li><b>Make use of the Microsoft Recovery and Support Assistant.<\/b> This is one very useful tool that Microsoft provides for troubleshooting Outlook issues for diagnosing and fixing Error 521.<\/li>\n<\/ol>\n\n\n\n<h3 class=\"wp-block-heading\">For Yahoo! users<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><b>Verify server connectivity.<\/b> 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.<\/li>\n\n\n\n<li><b>Verify your email settings.<\/b> Verify that your Yahoo! Mail account\u2019s SMTP settings are set correctly. The inability to send emails can be caused by incorrect SMTP settings.<\/li>\n<\/ol>\n\n\n\n<h3 class=\"wp-block-heading\">General troubleshooting steps<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><b>Ping the email server.<\/b> 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.<\/li>\n\n\n\n<li><b>Verify DNS records.<\/b> 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. If those MX records point to the wrong host entirely, rather than to a host that\u2019s simply refusing connections, you may actually be dealing with <a href=\"https:\/\/www.warmy.io\/blog\/smtp-email-error-512-how-to-resolve-solved\/\" target=\"_blank\" rel=\"noopener noreferrer\">SMTP Email Error 512<\/a> (bad destination address) or <a href=\"https:\/\/www.warmy.io\/blog\/how-to-fix-smtp-email-error-554-5-4-0-solved\/\" target=\"_blank\" rel=\"noopener noreferrer\">SMTP Email Error 554 5.4.0<\/a> (DNS routing failure) instead of 521.<\/li>\n\n\n\n<li><b>Examine the server logs.<\/b> 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.<\/li>\n<\/ol>\n\n\n\n<p>Tired of chasing bounce codes one at a time? <a href=\"https:\/\/app.warmy.io\/signup\" rel=\"noopener\" target=\"_blank\" rel=\"noopener noreferrer\">Start a free Warmy.io trial<\/a> and let Adeline AI monitor your domain\u2019s authentication and sender reputation continuously, so issues like Error 521 get flagged before they disrupt delivery.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"768\" src=\"https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2026\/06\/Warmy-Homepage-1024x768.png\" alt=\"Warmy Homepage\" class=\"wp-image-7290\" title=\"\" srcset=\"https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2026\/06\/Warmy-Homepage-1024x768.png 1024w, https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2026\/06\/Warmy-Homepage-300x225.png 300w, https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2026\/06\/Warmy-Homepage-768x576.png 768w, https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2026\/06\/Warmy-Homepage-1536x1152.png 1536w, https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2026\/06\/Warmy-Homepage.png 1920w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Advanced troubleshooting techniques for SMTP Email Error 521<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">Network diagnostics<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Network Analysis Tools.<\/strong> Utilize tools like <a href=\"https:\/\/www.wireshark.org\/\" rel=\"noopener\" target=\"_blank\" rel=\"noopener noreferrer\">Wireshark<\/a> 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.<\/li>\n\n\n\n<li><b>Advanced ping and traceroute tests. <\/b>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.\n<ul class=\"wp-block-list\">\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\n\n\n<h3 class=\"wp-block-heading\">Server configuration and logs<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\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\n\n\n<li>Verify SMTP server configuration. This includes checking port numbers, timeout settings, and whether the server is actually configured to listen for incoming connections.\n<ul class=\"wp-block-list\">\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\n\n\n<h3 class=\"wp-block-heading\">Security and access controls<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><b>Firewall and security rules.<\/b> Verify whether the firewall rules on the mail server as well as any other network devices are being used to block SMTP connections.\n<ul class=\"wp-block-list\">\n<li>Command to check firewall rules on Linux: <code>iptables -L<\/code><\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><b>SELinux policies. <\/b>If you are running a server with SELinux enabled, ensure that the security policies don\u2019t restrict the SMTP service from functioning properly.\n<ul class=\"wp-block-list\">\n<li>Command to check SELinux status: <code>sestatus<\/code><\/li>\n\n\n\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\n\n\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 &#8211; reducing downtime risks and preventing protective controls from inadvertently blocking legitimate mail flow.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Email server health checks<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><b>Check for blacklisting.<\/b> Use DNSBL (<a href=\"https:\/\/www.warmy.io\/blog\/dnsbl-blacklist-everything-you-need-to-know\/\" target=\"_blank\" rel=\"noopener noreferrer\">DNS-based Blackhole List<\/a>) checks to see if your server\u2019s IP is blacklisted, which could be causing other servers to reject connections.\n<ul class=\"wp-block-list\">\n<li>Related reading: <a href=\"https:\/\/www.warmy.io\/blog\/email-domain-ip-blacklist-removal-steps-to-delist-your-ip\/\" target=\"_blank\" rel=\"noopener noreferrer\">Email Domain &amp; IP Blacklist Removal: 5 Steps to Delist<\/a><\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><b>SMTP server performance monitoring.<\/b> 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.<\/li>\n<\/ol>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p>\ud83d\udca1 Pro Tip: A clean blacklist report doesn\u2019t rule out Error 521. Blacklisting and availability are separate problems &#8211; a host can be off every blacklist and still return 521 if it\u2019s down or configured to reject connections outright, so check both independently.<\/p>\n<\/blockquote>\n\n\n\n<h2 class=\"wp-block-heading\">Exploring additional variations of SMTP Email Error 522 and SMTP Email Error 523<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">Understanding SMTP Email Error 522<\/h3>\n\n\n\n<p>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 and cannot accept emails for the time being.Most often, this results in an NDR (non-delivery report) or bounce-back message being sent to the sender.<\/p>\n\n\n\n<p>The error message might look like this:<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\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\n\n\n<p>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.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Dealing with SMTP Email Error 523<\/h3>\n\n\n\n<p>SMTP Email Error 523, \u201cServer limit exceeded. Message too large,\u201d occurs when <b>the email sent is too large to be processed by the recipient\u2019s mail server. <\/b>This is often due to attachments that exceed the server\u2019s maximum allowed size for email messages.<\/p>\n\n\n\n<p>The sender will usually receive a notification that the email could not be delivered, with a message such as<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p>\u201cMessage size exceeds fixed maximum message size.\u201d<\/p>\n<\/blockquote>\n\n\n\n<p>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.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Use Warmy.io for proactive measures against SMTP Error 521 and others<\/h2>\n\n\n\n<figure class=\"wp-block-image size-full\"><img loading=\"lazy\" decoding=\"async\" width=\"861\" height=\"616\" src=\"https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2024\/05\/Screenshot_12.png\" alt=\"warmup performance\" class=\"wp-image-7126\" title=\"\" srcset=\"https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2024\/05\/Screenshot_12.png 861w, https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2024\/05\/Screenshot_12-300x215.png 300w, https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2024\/05\/Screenshot_12-768x549.png 768w\" sizes=\"auto, (max-width: 861px) 100vw, 861px\" \/><\/figure>\n\n\n\n<p>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.&nbsp;<\/p>\n\n\n\n<p>To avoid SMTP Error 521 or other similar errors, email senders may want to look into using <a href=\"https:\/\/warmy.io\" rel=\"noopener\" target=\"_blank\" rel=\"noopener noreferrer\">Warmy.io<\/a> to ensure reliable email deliverability. Warmy.io also offers a <a href=\"https:\/\/www.warmy.io\/free-tools\/email-deliverability-test\" target=\"_blank\" rel=\"noopener noreferrer\">free email deliverability test,<\/a> that can help discover and eliminate potential issues that can result in SMTP error (such as being classified as spam, for example).<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"623\" src=\"https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2024\/07\/Deliverability-test-1024x623.png\" alt=\"Deliverability test\" class=\"wp-image-7031\" title=\"\" srcset=\"https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2024\/07\/Deliverability-test-1024x623.png 1024w, https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2024\/07\/Deliverability-test-300x183.png 300w, https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2024\/07\/Deliverability-test-768x467.png 768w, https:\/\/www.warmy.io\/blog\/wp-content\/uploads\/2024\/07\/Deliverability-test.png 1060w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><\/figure>\n\n\n\n<p>Warmy.io has additional free tools that can help you generate <a href=\"https:\/\/www.warmy.io\/free-tools\/spf-generator\" target=\"_blank\" rel=\"noopener noreferrer\">SPF<\/a> and <a href=\"https:\/\/www.warmy.io\/free-tools\/dmarc-generator\" target=\"_blank\" rel=\"noopener noreferrer\">DMARC<\/a> 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 &#8211; day in and day out.<\/p>\n\n\n\n<p>Don&#8217;t wait for the next rejection to find out your authentication has drifted out of sync. <a href=\"https:\/\/www.warmy.io\/book-a-demo\" target=\"_blank\" rel=\"noopener noreferrer\">Book a demo<\/a> and let Warmy handle the monitoring, alerts, and record upkeep for you, so your SPF and DMARC stay correctly configured and your emails keep reaching the inbox without the manual checkups.<\/p>\n\n\n\n<figure class=\"wp-block-embed is-type-video is-provider-youtube wp-block-embed-youtube wp-embed-aspect-16-9 wp-has-aspect-ratio\"><div class=\"wp-block-embed__wrapper\">\n<iframe title=\"How Warmy.io Works in 2026\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/smB4UXIV_Xk?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe>\n<\/div><\/figure>\n","protected":false},"excerpt":{"rendered":"<p>Email remains central to both personal and professional communication, which is exactly why protocol-level failures like Simple Mail Transfer Protocol (SMTP) errors are so disruptive &#8211; they stop that communication cold. SMTP Error 521 means the recipient\u2019s mail server is refusing to accept any incoming mail at all &#8211; the connection opens, then immediately closes [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":7885,"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":4,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/posts\/3869\/revisions"}],"predecessor-version":[{"id":7888,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/posts\/3869\/revisions\/7888"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.warmy.io\/blog\/wp-json\/wp\/v2\/media\/7885"}],"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}]}}