SMTP response received
450 4.2.0 Recipient Address Rejected Greylisted: Causes and FixYou received this code because a receiving mail server refused delivery with code 450 4.2.0. This is a transient (4.x.x) failure. The specific cause depends on the exact enhanced status code and diagnostic text returned by the receiver โ this guide covers the general resolution pattern.
๐ In this guide
๐ค Who sees this code
- Sender debugging a 450 4.2.0 bounce in their pipeline
- Ops team investigating an unfamiliar SMTP response code
- Developer building bounce processing logic
๐ง What this guide fixes
- Interpret the exact meaning of 450 4.2.0 correctly
- Distinguish sender-side from receiver-side causes
- Identify the right remediation path
๐ The 4.2.x transient mailbox family
421 4.2.1Mailbox busy ยท transient
421 4.2.2Mailbox full ยท transient
550 5.2.1Permanent version ยท disabled
550 5.2.2Permanent version ยท full
โก Do these 3 things first (before diving deeper)
Read the diagnostic-code text after 450 4.2.0
The numeric code tells you the failure class but the wording after it points at the specific cause. Extract it from the Diagnostic-Code field of the bounce message. Different receivers use different wording for the same underlying cause, so the text is what actually guides your remediation rather than the numeric code alone.
Determine whether the failure is sender-side or receiver-side
Some 450 4.2.0 variants originate from your outbound infrastructure (relay refusal, credentials, configuration). Others originate from the receiver (policy, reputation, address). The two require different debugging paths. If the bounce came from your own SMTP server, it is sender-side. If it came from the recipient domain, it is receiver-side.
Check the surrounding context for repeat patterns
One 450 4.2.0 is a data point. Ten 450 4.2.0s from the same batch, same domain, or same time window is a pattern that points at the specific cause. Look at your recent outbound logs for other 450 4.2.0s and identify what they share: a common recipient domain, a common sending IP, a common template.
Common causes of 450 4.2.0
- Sender-side configuration or infrastructure problem. Your outbound SMTP server or application configuration produced a state the receiver refuses. This includes wrong credentials, missing SMTP AUTH, IP not on the allowlist, misconfigured relay hostname, or a domain not in Accepted Domains. See our 550 5.7.1 Relaying denied guide for the deep-dive on sender-side relay failures. Check application logs and outbound server config first when the bounce comes back quickly after send.
- Receiver-side policy enforcement. The receiving server evaluated your message against its policy rules (authentication, reputation, content filtering, blocklist) and decided to refuse. This is the most common cause when the bounce arrives shortly after RCPT TO from a major receiver like Gmail, Outlook, or Yahoo. Our 550 5.4.1 Office 365 walkthrough covers the Microsoft-specific policy path in depth. The remediation depends on which specific policy triggered.
- Recipient address itself is invalid or unavailable. The address does not exist, was deleted, or is disabled at the receiving server. Codes in the 5.1.x range indicate addressing problems (see our 550 5.1.1 User unknown playbook); 5.2.x indicates mailbox problems (see our 550 5.2.1 Inactive account guide for the inactive-mailbox variant). If the receiver returns 5.1.x or 5.2.x wording alongside 450 4.2.0, suppression rather than remediation is the correct action.
- Reputation or blocklist trigger. Your sending IP or domain has poor reputation with the receiver or is on a blocklist consulted by that receiver. This produces policy rejections even when authentication is technically valid. Check reputation dashboards (Postmaster Tools, SNDS, Sender Score) and blocklist status through MXToolbox or MultiRBL. Our 550 5.7.1 Spamhaus and 550 5.7.1 Barracuda playbooks cover the specific delisting flows for the two most-consulted enterprise blocklists.
- Content or size violation. The message content triggered a filter (spam-like language, suspicious links, malformed MIME) or exceeded a size limit at the receiver. Content-driven rejections are less common than authentication or reputation rejections but do occur, especially with automated transactional mail that has grown over time.
How to fix 450 4.2.0, step by step
- Extract the exact diagnostic text from the bounce. The wording after 450 4.2.0 is your diagnostic starting point. Save it verbatim, including any bracketed tags or reference codes. Different receivers use different phrasing for the same issue, and the exact wording maps to different remediation paths.
- Identify sender-side vs receiver-side origin. If the bounce came from your own SMTP infrastructure (Message-Id or source pointing at your relay), the fix is on your side: credentials, config, allowlist. If it came from the receiving domain, the fix depends on the receiver policy or reputation.
- Address the identified root cause. For sender-side: fix credentials, enable SMTP AUTH, correct hostnames, or add missing IPs to allowlists. For receiver-side reputation: monitor dashboards, reduce complaint rate, warm up affected IPs. For receiver-side authentication: fix SPF, DKIM, and DMARC alignment. For content or size: revise the affected message.
- Verify the fix with a controlled test send. Send a test message from the corrected path to a known-good recipient at the same receiver. Check the delivery status carefully. Do not resume production traffic until the test succeeds and any relevant metrics (auth results header, reputation dashboards) confirm the fix.
- If the issue persists, contact the receiver postmaster. When you have systematically addressed sender-side possibilities and the bounces continue, the last recourse is contacting the receiver directly. Provide your Message-ID, exact bounce text, timestamps, and evidence of your remediation attempts. Major receivers (Gmail, Microsoft, Yahoo) have documented postmaster escalation processes.
๐ How each provider sends this exact code
โ How this code differs from adjacent ones
Related codes in same familyTransient version (4.x.x)Cross-family variantsPrevention going forward
Preventing 450 4.2.0 rests on the general discipline of good sender operations: clean lists, authenticated sending, monitored reputation, and prompt response to early warnings.
- Publish and maintain SPF, DKIM, and DMARC records with proper alignment. Authentication is the baseline that receivers use to distinguish legitimate senders from spoofers.
- Monitor reputation dashboards weekly across all major receivers. Reputation drops precede rejections by days or weeks and give you time to correct behavior.
- Validate addresses before sending to reduce bounce-driven reputation damage. Address validation catches most invalid addresses before they generate bounces.
- Register for provider Feedback Loops to receive complaints in real time. Complainers should be suppressed on the first complaint, not on the third campaign.
- Warm up new sending IPs following a proper 30/60/90 schedule. Sudden volume from unknown IPs is the fastest way to trigger reputation problems.
Frequently asked questions about 450 4.2.0
What does 450 4.2.0 mean exactly?
Should I retry after receiving 450 4.2.0?
Should I suppress the recipient after 450 4.2.0?
How is 450 4.2.0 different from other codes in the same class?
Does 450 4.2.0 affect my sender reputation?
How long does resolution take?
Stop 450 4.2.0 bounces before they happen.
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๐ Deep dive on related codes
550 5.4.1Office 365 tenant policyRecipient address rejected at Microsoft โ auth or reputation cause
550 5.1.1User unknownRecipient address does not exist โ suppress immediately
550 5.7.1Relaying deniedSender-side SMTP AUTH failure โ fix your outbound config
550 5.7.1Blocked by SpamhausSpamhaus SBL/PBL/CSS/XBL delisting playbook
About the Author
Alaa ยท LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. Iโve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
About SMTPedia
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