SMTP error 450: What it means and how to fix it
SMTP error 450 overview:
- SMTP 450 is a temporary deferral (the mailbox/server can’t accept the message yet)
- Let your system retry normally since it often delivers on the next attempt
- Use the full SMTP text and 4.x.x code (if available) to pinpoint the exact reason for deferral
- Common triggers are receiving-side limits like greylisting, rate caps, or congestion
- Escalate only if it persists by sharing the full bounce text with the recipient
SMTP error 450 indicates the recipient’s mailbox is temporarily unavailable. The receiving server can’t deliver your message right now, but it’s usually short-lived. Your server will retry delivery automatically.
Retries often resolve a 450. Permanent authentication errors are a different problem. Sendmarc helps you keep SPF, DKIM, and DMARC correctly configured across every sending source, reducing deliverability issues.
What SMTP error 450 means
An SMTP 450 error is usually returned with the status text “Requested mail action not taken: mailbox unavailable.” The mailbox exists, but the receiving server can’t accept the message right now. Because it’s a temporary response, the server expects the sender to retry.
Common causes include the recipient mailbox being over quota or temporarily locked (for example, during maintenance).
Common causes of SMTP error 450
- Greylisting: The receiving server temporarily defers first-time senders as an anti-spam measure
- Rate limiting: The receiving server temporarily limits how many messages it will accept from a sender or domain
- Mailbox locking: The recipient’s mailbox is temporarily inaccessible (for example, during maintenance)
- Server congestion: The receiving server is busy or processing unusually high inbound volume
- Spam filtering: The receiving server flagged the message as suspicious and deferred it
- Authentication checks: The receiving server can’t validate the authentication at that moment
Provider-specific 450 errors
Google (Gmail, Google Workspace)
| Error message | Meaning |
|---|---|
| 450 4.2.1 – The user you are trying to contact is receiving mail at a rate that prevents additional messages | Recipient is receiving email too quickly; retry later |
Yahoo (Yahoo, AOL, Verizon)
| Error message | Meaning |
|---|---|
| 450 – User is receiving mail too quickly | Recipient-side deferral due to high inbound volume; retry later |
Microsoft (Outlook.com)
| Error message | Meaning |
|---|---|
| 450 4.7.3 – Organization queue quota exceeded | The recipient domain is under heavy inbound load; retry later |
Yandex
| Error message | Meaning |
|---|---|
| 450 4.2.1 – The recipient message rate limit exceeded | Too many messages to this recipient in a short period; retry later |
ImprovMX
| Error message | Meaning |
|---|---|
| 450 4.2.1 – Please try again later | Temporary deferral by the receiving service; retry later |
Rackspace
| Error message | Meaning |
|---|---|
| 450 4.7.1 – Client host has no PTR record | The reverse DNS record for the IP address is missing |
| 450 4.4.1 – The recipient’s server was temporarily unavailable | Temporary receiving-side issue; retry later |
| 450 4.2.1 – Mailbox received too much email in a short period of time | Temporary deferral due to high inbound volume; retry later |
| 450 4.4.6 – Routing loop detected | A routing loop is suspected; routing/configuration needs review |
| 450 4.4.0 – Temporary DMARC DNS lookup failure | DMARC (Domain-based Message Authentication, Reporting, and Conformance) check couldn’t complete due to a temporary issue |
Email Security.cloud
| Error message | Meaning |
|---|---|
| 450 – Requested action aborted | Temporarily deferred by the Email Security.cloud filtering service |
How to fix SMTP error 450
Step 1: Read the full SMTP response
A 450 error is a temporary deferral, but the extra text (and sometimes an enhanced status like 4.2.1, 4.4.1, 4.7.1) tells you the specific reason and what to do next.
Step 2: Let automatic retries run
Because 450 errors are temporary, the correct first action is usually to let your email system retry. Whether it succeeds depends on the underlying cause (greylisting, load, locking, etc.).
Step 3: Check sending rate
If the 450 wording indicates “too much email,” “too quickly,” “rate,” or similar:
- Reduce burst sending to that mailbox
- Add throttling in your sending platform
- Spread sends over a longer window
Step 4: Fix reverse DNS
If the response includes “no PTR record” or similar:
- Ensure the sending IP has a PTR record
- Ensure the PTR hostname resolves back to the same IP
Step 5: Review email authentication
A 450 response can reference DMARC. In that case, confirm your DMARC record exists and is published correctly.
DMARC record example:
| Host | Type | Value |
|---|---|---|
_dmarc.yourdomain.com | TXT | v=DMARC1; p=reject; rua=mailto:dmarc-reports@yourdomain.com; fo=1; |
Step 6: Escalate when it’s persistent
If retries continue to fail for one recipient while other emails deliver:
- Share the full SMTP error/bounce text you received with the recipient
- Ask them to confirm whether their mailbox is under temporary restriction
Improve deliverability and reduce risk with Sendmarc
SMTP 450 errors are usually temporary and often clear with retries. But strong email authentication still matters because it helps prevent permanent failures that won’t clear on their own.
Sendmarc helps you:
- Prevent ongoing authentication failures by keeping SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC correctly configured and aligned across all senders
- Improve visibility and control across every email-sending tool and vendor – so you can quickly identify unknown or unauthorized senders
- Protect brand reputation against phishing, spoofing, impersonation, and lookalike domains
- Detect and respond to compromised employee accounts and suspicious sending activity faster
- Reduce issues caused by misconfigured senders across billing, notifications, and marketing streams
- Maintain continuous improvements without increasing internal workload through monitoring, reporting, and hands-on support
- Strengthen governance and audit readiness with credible reporting and clear authentication standards across regions and teams
Learn how Sendmarc helps you standardize and monitor SPF, DKIM, and DMARC across every sender.
SMTP error 450 FAQs
How is a 450 error different from a 550 error?
A 450 error is a temporary (4xx) SMTP response, which means the receiving server is saying “try again later,” so your sending server will usually retry delivery automatically. A 550 error is a permanent (5xx) SMTP response, which means the receiving server is rejecting the message and it won’t be delivered unless something changes.
Should I resend emails that get 450 errors?
No, you generally shouldn’t resend emails after a 450 error because a 450 error is temporary, and your email server will typically retry automatically. Manually resending can create duplicate attempts and may increase the chance of rate limiting.
Why do I get 450 errors from some providers but not others?
You can get 450 errors from some providers but not others because each provider enforces different anti-spam and traffic policies. Some providers use more aggressive greylisting, tighter rate limits, or stricter reputation requirements.