{"id":5295,"date":"2026-03-17T01:07:19","date_gmt":"2026-03-17T01:07:19","guid":{"rendered":"https:\/\/emailwarmup.com\/blog\/?p=5295"},"modified":"2026-03-17T01:07:21","modified_gmt":"2026-03-17T01:07:21","slug":"how-to-fix-450-4-4-318-suspicious-remote-server-error","status":"publish","type":"post","link":"https:\/\/emailwarmup.com\/blog\/email-errors\/how-to-fix-450-4-4-318-suspicious-remote-server-error\/","title":{"rendered":"How To Fix 450 4.4.318 |  Suspicious Remote Server Error"},"content":{"rendered":"\n<figure class=\"wp-block-image size-full\"><img fetchpriority=\"high\" decoding=\"async\" width=\"3112\" height=\"1980\" src=\"https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg\" alt=\"450 4.4.318\" class=\"wp-image-5296\"\/><\/figure>\n\n\n\n<p>The 450 4.4.318 error means Exchange detected a suspicious termination of a connection during SMTP transmission \u2014 typically caused by a firewall&#8217;s SMTP inspection breaking the session.&nbsp;<\/p>\n\n\n\n<p>Fix it by disabling SMTP protocol inspection on your firewall (the most common culprit), increasing maximum header size limits, and verifying PTR records match your Exchange server&#8217;s FQDN.<\/p>\n\n\n\n<p>Unlike authentication or content rejections, the 4.4.318 code signals <em>infrastructure interference<\/em> \u2014 something between your server and the recipient dropped the connection mid-transmission.&nbsp;<\/p>\n\n\n\n<p>Microsoft Exchange environments encounter this error frequently when security appliances (Meraki, WatchGuard, SonicWall) inspect SMTP traffic and misinterpret legitimate patterns as threats.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Quick skim \u2014 4.4.318 error overview<\/h2>\n\n\n\n<p>The 450 4.4.318 error indicates connection-level interference rather than message rejection.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td>Attribute<\/td><td>Details<\/td><\/tr><tr><td>Error code<\/td><td>450 4.4.318<\/td><\/tr><tr><td>Category<\/td><td>Transport\/connection failure<\/td><\/tr><tr><td>Meaning<\/td><td>Connection terminated unexpectedly (flagged as suspicious)<\/td><\/tr><tr><td>Severity<\/td><td>Temporary (requires infrastructure fix)<\/td><\/tr><tr><td>Common causes<\/td><td>Firewall SMTP inspection, header size limits, PTR mismatch<\/td><\/tr><tr><td>Fix approach<\/td><td>Disable SMTP inspection \u2192 adjust limits \u2192 verify DNS<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">What does <em>Suspicious Remote Server Error<\/em> mean?<\/h2>\n\n\n\n<p>Exchange flags connections as &#8220;suspicious&#8221; when the remote server&#8217;s behavior deviates from expected SMTP patterns. The receiving server might terminate abruptly, send malformed responses, or timeout unexpectedly \u2014 all symptoms that <em>often originate from security appliances<\/em> rather than the actual mail server.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Why firewalls cause this<\/h3>\n\n\n\n<p>SMTP inspection features analyze email traffic for threats. However, inspection engines sometimes:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Modify SMTP headers (breaking signatures or exceeding size limits)<\/li>\n\n\n\n<li>Introduce latency (triggering timeouts)<\/li>\n\n\n\n<li>Misinterpret legitimate patterns as attacks<\/li>\n\n\n\n<li>Terminate connections they can&#8217;t fully analyze<\/li>\n<\/ul>\n\n\n\n<p>The firewall <em>thinks<\/em> it&#8217;s protecting you \u2014 but it&#8217;s actually breaking mail flow (an ironic security failure).<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Exchange&#8217;s perspective<\/h3>\n\n\n\n<p>Exchange sees the connection terminate without proper SMTP closure. From Exchange&#8217;s viewpoint, the remote server behaved suspiciously. In reality, a middlebox (firewall, proxy, security appliance) caused the disruption.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Why does this error occur?<\/h2>\n\n\n\n<p>Several infrastructure configurations trigger the Suspicious RemoteServer Error.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Firewall SMTP inspection<\/h3>\n\n\n\n<p>The primary cause \u2014 security appliances inspecting port 25 traffic:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><em>Meraki<\/em> \u2014 Content filtering with SMTP inspection enabled<\/li>\n\n\n\n<li><em>WatchGuard<\/em> \u2014 SMTP proxy\/inspection features<\/li>\n\n\n\n<li><em>SonicWall<\/em> \u2014 Email security services<\/li>\n\n\n\n<li><em>Cisco ASA<\/em> \u2014 ESMTP inspection<\/li>\n<\/ul>\n\n\n\n<p>Disabling SMTP-specific inspection (while keeping general firewall rules) typically resolves the error immediately.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Header size limits<\/h3>\n\n\n\n<p>Firewalls enforce maximum header sizes. Emails exceeding limits get truncated or dropped:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Marketing emails with extensive tracking headers<\/li>\n\n\n\n<li>Messages with large recipient lists (CC\/BCC)<\/li>\n\n\n\n<li>DKIM signatures on complex messages<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">PTR record mismatch<\/h3>\n\n\n\n<p>Exchange validates sender infrastructure. Mismatched PTR records trigger suspicion:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>PTR points to old server hostname<\/li>\n\n\n\n<li>Forward DNS doesn&#8217;t resolve back to the sending IP<\/li>\n\n\n\n<li>The recent migration left stale records<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Disk space exhaustion<\/h3>\n\n\n\n<p>Low disk space on Exchange servers causes transport failures:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Exchange requires 15%+ free space on transport volumes<\/li>\n\n\n\n<li>Below threshold, the transport service behaves erratically<\/li>\n\n\n\n<li>Connections may drop mid-transmission<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">How do you fix 450 4.4.318?<\/h2>\n\n\n\n<p>Address firewall configuration first (resolves most cases), then verify DNS and Exchange settings.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Disable SMTP inspection<\/h3>\n\n\n\n<p>The most common fix \u2014 turn off SMTP-specific inspection on your firewall:<\/p>\n\n\n\n<h4 class=\"wp-block-heading\">Meraki<\/h4>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Navigate to Security &amp; SD-WAN \u2192 Threat Protection<\/li>\n\n\n\n<li>Disable protocol-specific inspection for SMTP<\/li>\n\n\n\n<li>Allow port 25 traffic without deep inspection<\/li>\n<\/ul>\n\n\n\n<h4 class=\"wp-block-heading\">WatchGuard<\/h4>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Access Fireware Web UI<\/li>\n\n\n\n<li>Locate SMTP Proxy settings<\/li>\n\n\n\n<li>Disable SMTP proxy or switch to packet filter mode<\/li>\n\n\n\n<li>Review content inspection policies<\/li>\n<\/ul>\n\n\n\n<h4 class=\"wp-block-heading\">SonicWall<\/h4>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Navigate to Security Services \u2192 Email Security<\/li>\n\n\n\n<li>Disable inbound\/outbound SMTP inspection<\/li>\n\n\n\n<li>Maintain general firewall rules (don&#8217;t disable firewall entirely)<\/li>\n<\/ul>\n\n\n\n<h4 class=\"wp-block-heading\">General approach<\/h4>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Identify which security appliance sits between Exchange and internet<\/li>\n\n\n\n<li>Locate SMTP\/email-specific inspection settings<\/li>\n\n\n\n<li>Disable inspection for SMTP traffic<\/li>\n\n\n\n<li>Test mail flow immediately after changes<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Increase header size limits<\/h3>\n\n\n\n<p>If SMTP inspection must remain enabled, adjust limits:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Increase maximum allowed header size (default often too small)<\/li>\n\n\n\n<li>Set limit above 32KB minimum (Gmail requires this)<\/li>\n\n\n\n<li>Allow larger messages through the inspection engine<\/li>\n<\/ul>\n\n\n\n<p>Configuration location varies by firewall vendor \u2014 consult documentation for specific settings.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Verify DNS records<\/h3>\n\n\n\n<p>Confirm PTR and forward DNS alignment:<\/p>\n\n\n\n<h4 class=\"wp-block-heading\">PTR record<\/h4>\n\n\n\n<p>bash<\/p>\n\n\n\n<p>dig -x YOUR.EXCHANGE.IP<\/p>\n\n\n\n<p>Result should return your Exchange server&#8217;s FQDN (exactly as configured in send connectors).<\/p>\n\n\n\n<h4 class=\"wp-block-heading\">Forward DNS<\/h4>\n\n\n\n<p>bash<\/p>\n\n\n\n<p>dig A mail.yourdomain.com<\/p>\n\n\n\n<p>Result should return your Exchange server&#8217;s IP. Mismatches cause suspicion flags at receiving servers.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Check Exchange health<\/h3>\n\n\n\n<p>Verify Exchange isn&#8217;t contributing to the problem:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Confirm 15%+ free disk space on transport volumes<\/li>\n\n\n\n<li>Restart the Microsoft Exchange Transport service<\/li>\n\n\n\n<li>Clear stuck messages from the queue<\/li>\n\n\n\n<li>Review connector timeout settings<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Whitelist EOP IPs (hybrid)<\/h3>\n\n\n\n<p>For hybrid Exchange environments:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Exchange Online Protection (EOP) sends through specific IP ranges<\/li>\n\n\n\n<li>Firewall must allow EOP IPs without inspection<\/li>\n\n\n\n<li>Microsoft publishes current IP ranges in documentation<\/li>\n\n\n\n<li>Add ranges to firewall allow lists<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Create a dedicated connector<\/h3>\n\n\n\n<p>As a targeted workaround:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Create a send connector for problematic destination domains<\/li>\n\n\n\n<li>Configure less restrictive settings on the dedicated connector<\/li>\n\n\n\n<li>Route specific traffic through a cleaner path<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">How do you prevent the 4.4.318 error?<\/h2>\n\n\n\n<p>Infrastructure hygiene prevents most SuspiciousRemoteServerError occurrences.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Audit security appliances<\/h3>\n\n\n\n<p>Review SMTP handling across your network stack:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Document which devices inspect email traffic<\/li>\n\n\n\n<li>Test mail flow after any firewall changes<\/li>\n\n\n\n<li>Maintain baseline configuration for quick rollback<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Monitor transport health<\/h3>\n\n\n\n<p>Set alerts for early warning:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Queue buildup on Exchange servers<\/li>\n\n\n\n<li>450 error rate increases<\/li>\n\n\n\n<li>Disk space approaching thresholds<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Coordinate with recipients<\/h3>\n\n\n\n<p>If the receiving organization&#8217;s firewall causes issues (you&#8217;re the sender):<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Contact their IT department<\/li>\n\n\n\n<li>Explain the error and likely cause<\/li>\n\n\n\n<li>Request that they review their SMTP inspection settings<\/li>\n<\/ul>\n\n\n\n\n<!-- EmailWarmup \u2014 Email Errors Campaign CTA Section -->\n<section id=\"ew-email-errors-consult\" aria-label=\"Help fixing email errors\">\n  <style>\n    \/* Scoped, theme-safe resets *\/\n    #ew-email-errors-consult, #ew-email-errors-consult *{\n      box-sizing:border-box;\n      -webkit-hyphens:none!important; 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}\n    #ew-email-errors-consult .btn:active{ transform:translateY(1px); }\n\n    @media (max-width:640px){\n      #ew-email-errors-consult .title{ font-size:24px; }\n    }\n  <\/style>\n\n  <div class=\"intro\">\n    <div class=\"eyebrow\">Need help fixing an email error?<\/div>\n    <h2 class=\"title\">Still stuck after trying the fix?<\/h2>\n    <p class=\"subtitle\">\n      Some email errors are easy to clear. Others point to deeper deliverability issues involving authentication, sender reputation, blacklisting, routing, or mailbox provider policy. If you would rather have an expert review it, speak with an\n      <a href=\"https:\/\/emailwarmup.com\/email-deliverability-consultant\">email delieverability consultant<\/a>\n      for free and we can help diagnose the issue and fix it on your behalf.\n    <\/p>\n  <\/div>\n\n  <div class=\"tiles\" role=\"list\">\n    <article class=\"tile t1\" role=\"listitem\">\n      <div class=\"icon\" aria-hidden=\"true\">\n        <svg width=\"18\" height=\"18\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\">\n          <path d=\"M21 15a2 2 0 0 1-2 2H7l-4 4V5a2 2 0 0 1 2-2h14a2 2 0 0 1 2 2z\"><\/path>\n        <\/svg>\n      <\/div>\n      <div>\n        <span class=\"h3like\">Root cause analysis<\/span>\n        <p>We look beyond the error message itself to find what is actually breaking delivery, trust, or inbox placement.<\/p>\n      <\/div>\n    <\/article>\n\n    <article class=\"tile t2\" role=\"listitem\">\n      <div class=\"icon\" aria-hidden=\"true\">\n        <svg width=\"18\" height=\"18\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\">\n          <path d=\"M20 6L9 17l-5-5\"><\/path>\n        <\/svg>\n      <\/div>\n      <div>\n        <span class=\"h3like\">Technical fixes handled for you<\/span>\n        <p>From SPF, DKIM, and DMARC to blacklist cleanup, DNS alignment, and sending setup, we can guide or implement the fix.<\/p>\n      <\/div>\n    <\/article>\n\n    <article class=\"tile t3\" role=\"listitem\">\n      <div class=\"icon\" aria-hidden=\"true\">\n        <svg width=\"18\" height=\"18\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\">\n          <path d=\"M12 20V10\"><\/path>\n          <path d=\"M18 20V4\"><\/path>\n          <path d=\"M6 20v-4\"><\/path>\n        <\/svg>\n      <\/div>\n      <div>\n        <span class=\"h3like\">Deliverability-first review<\/span>\n        <p>We assess whether the error is part of a bigger pattern hurting opens, replies, and overall campaign performance.<\/p>\n      <\/div>\n    <\/article>\n\n    <article class=\"tile t4\" role=\"listitem\">\n      <div class=\"icon\" aria-hidden=\"true\">\n        <svg width=\"18\" height=\"18\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\">\n          <path d=\"M12 12c2.8 0 5-2.2 5-5s-2.2-5-5-5-5 2.2-5 5 2.2 5 5 5z\"><\/path>\n          <path d=\"M2 22c1.5-4 5.5-6 10-6s8.5 2 10 6\"><\/path>\n        <\/svg>\n      <\/div>\n      <div>\n        <span class=\"h3like\">Free expert consultation<\/span>\n        <p>Talk to a real deliverability expert, get honest guidance, and see the next best step without pressure or upsells.<\/p>\n      <\/div>\n    <\/article>\n  <\/div>\n\n  <div class=\"bottom\">\n    <p>\n      <strong>When should you book a consultation?<\/strong> If the error keeps coming back, affects multiple mailboxes or domains, started after an ESP or DNS change, or is tied to spam placement, low inboxing, high bounce rates, or authentication failures, it is usually faster to get an expert involved early.\n    <\/p>\n  <\/div>\n\n  <div class=\"cta\">\n    <a class=\"btn\" href=\"https:\/\/emailwarmup.com\/email-deliverability-consultant\" target=\"_blank\" rel=\"noopener\">\n      Talk to a deliverability expert for free\n      <svg width=\"18\" height=\"18\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\" aria-hidden=\"true\">\n        <path d=\"M5 12h14\"><\/path>\n        <path d=\"M12 5l7 7-7 7\"><\/path>\n      <\/svg>\n    <\/a>\n  <\/div>\n<\/section>\n\n\n\n\n\n<h2 class=\"wp-block-heading\">Frequently asked questions<\/h2>\n\n\n\n<p>Here are some commonly asked questions about this error:<\/p>\n\n\n\n<div class=\"schema-faq wp-block-yoast-faq-block\"><div class=\"schema-faq-section\" id=\"faq-question-1773709576981\"><strong class=\"schema-faq-question\">Why does my firewall break email when it&#8217;s supposed to protect it?<\/strong> <p class=\"schema-faq-answer\">SMTP inspection attempts to scan email content for threats. However, inspection engines often struggle with modern email complexity \u2014 large headers, TLS negotiation, and extended SMTP features. The inspection causes more problems than it solves for organizations with proper email security (SPF\/DKIM\/DMARC, email filtering services). Disabling SMTP inspection while maintaining other security controls is usually safe.<\/p> <\/div> <div class=\"schema-faq-section\" id=\"faq-question-1773709584267\"><strong class=\"schema-faq-question\">Is this error on my side or the recipient&#8217;s side?<\/strong> <p class=\"schema-faq-answer\">The 450 4.4.318 error typically indicates the <em>receiving<\/em> infrastructure behaved unexpectedly (from Exchange&#8217;s perspective). However, your own firewall might be the cause if it inspects outbound SMTP. Check both sides \u2014 your egress path and their ingress path.<\/p> <\/div> <div class=\"schema-faq-section\" id=\"faq-question-1773709590396\"><strong class=\"schema-faq-question\">Should I just disable my firewall?<\/strong> <p class=\"schema-faq-answer\">No. Disable <em>SMTP inspection specifically<\/em>, not your entire firewall. General firewall rules (blocking unauthorized ports, preventing intrusions) remain essential. SMTP inspection is a specific feature that often causes more problems than it prevents.<\/p> <\/div> <\/div>\n","protected":false},"excerpt":{"rendered":"<p>The 450 4.4.318 error means Exchange detected a suspicious termination of a connection during SMTP transmission \u2014 typically caused by a firewall&#8217;s SMTP inspection breaking the session.&nbsp; Fix it by disabling SMTP protocol inspection on your firewall (the most common culprit), increasing maximum header size limits, and verifying PTR records match your Exchange server&#8217;s FQDN. [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":5296,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_uag_custom_page_level_css":"","footnotes":""},"categories":[38],"tags":[],"class_list":["post-5295","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-email-errors"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v25.0 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>How To Fix 450 4.4.318 | Suspicious Remote Server Error<\/title>\n<meta name=\"description\" content=\"The 450 4.4.318 error means Exchange detected a suspicious termination of a connection during SMTP transmission. Let&#039;s fix!\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/emailwarmup.com\/blog\/email-errors\/how-to-fix-450-4-4-318-suspicious-remote-server-error\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How To Fix 450 4.4.318 | Suspicious Remote Server Error\" \/>\n<meta property=\"og:description\" content=\"The 450 4.4.318 error means Exchange detected a suspicious termination of a connection during SMTP transmission. 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The inspection causes more problems than it solves for organizations with proper email security (SPF\/DKIM\/DMARC, email filtering services). Disabling SMTP inspection while maintaining other security controls is usually safe.","inLanguage":"en-US"},"inLanguage":"en-US"},{"@type":"Question","@id":"https:\/\/emailwarmup.com\/blog\/email-errors\/how-to-fix-450-4-4-318-suspicious-remote-server-error\/#faq-question-1773709584267","position":2,"url":"https:\/\/emailwarmup.com\/blog\/email-errors\/how-to-fix-450-4-4-318-suspicious-remote-server-error\/#faq-question-1773709584267","name":"Is this error on my side or the recipient's side?","answerCount":1,"acceptedAnswer":{"@type":"Answer","text":"The 450 4.4.318 error typically indicates the <em>receiving<\/em> infrastructure behaved unexpectedly (from Exchange's perspective). However, your own firewall might be the cause if it inspects outbound SMTP. Check both sides \u2014 your egress path and their ingress path.","inLanguage":"en-US"},"inLanguage":"en-US"},{"@type":"Question","@id":"https:\/\/emailwarmup.com\/blog\/email-errors\/how-to-fix-450-4-4-318-suspicious-remote-server-error\/#faq-question-1773709590396","position":3,"url":"https:\/\/emailwarmup.com\/blog\/email-errors\/how-to-fix-450-4-4-318-suspicious-remote-server-error\/#faq-question-1773709590396","name":"Should I just disable my firewall?","answerCount":1,"acceptedAnswer":{"@type":"Answer","text":"No. Disable <em>SMTP inspection specifically<\/em>, not your entire firewall. General firewall rules (blocking unauthorized ports, preventing intrusions) remain essential. SMTP inspection is a specific feature that often causes more problems than it prevents.","inLanguage":"en-US"},"inLanguage":"en-US"}]}},"uagb_featured_image_src":{"full":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",3112,1980,false],"thumbnail":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",150,95,false],"medium":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",300,191,false],"medium_large":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",768,489,false],"large":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",1024,652,false],"1536x1536":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",1536,977,false],"2048x2048":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",2048,1303,false],"profile_24":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",24,15,false],"profile_48":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",48,31,false],"profile_96":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",96,61,false],"profile_150":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",150,95,false],"profile_300":["https:\/\/emailwarmup.com\/blog\/wp-content\/uploads\/2026\/03\/450-4.4.318_11zon.jpg",300,191,false]},"uagb_author_info":{"display_name":"Daniyal Dehleh","author_link":"https:\/\/emailwarmup.com\/blog\/author\/daniyaldehleh\/"},"uagb_comment_info":0,"uagb_excerpt":"The 450 4.4.318 error means Exchange detected a suspicious termination of a connection during SMTP transmission \u2014 typically caused by a firewall&#8217;s SMTP inspection breaking the session.&nbsp; Fix it by disabling SMTP protocol inspection on your firewall (the most common culprit), increasing maximum header size limits, and verifying PTR records match your Exchange server&#8217;s FQDN.&hellip;","_links":{"self":[{"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/posts\/5295","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/comments?post=5295"}],"version-history":[{"count":1,"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/posts\/5295\/revisions"}],"predecessor-version":[{"id":5297,"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/posts\/5295\/revisions\/5297"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/media\/5296"}],"wp:attachment":[{"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/media?parent=5295"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/categories?post=5295"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/emailwarmup.com\/blog\/wp-json\/wp\/v2\/tags?post=5295"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}