Web Technician OnlineWhat does this mean?
450 is a temporary rejection associated with the requested mail action. Provider text may identify a busy mailbox, policy check or another temporary condition. It is different from assuming the address is permanently invalid, so use the whole reply before deleting a contact.
What should I check first?
Record the recipient domain, time, message ID and complete reply. Confirm the sending server still has the message queued. Compare a single known recipient with another mailbox at the same provider only when appropriate; do not send a burst of test messages.
How can I diagnose the cause?
Check provider status and queue history for recurring deferrals. If the response mentions mailbox quota, the recipient or administrator may need to free storage. If it names a sender policy, inspect that precise condition with your sending provider rather than changing the recipient’s domain configuration.
How do I fix it safely?
Let normal retries continue for a documented temporary condition. Correct a confirmed account limit or authentication problem using the relevant provider’s instructions. Keep a copy before editing sender settings, and do not replace MX records merely because one mailbox returns a temporary code.
Verify the fix and know when to contact your provider
Verify whether delivery eventually succeeded before resending. Ask your sending provider to trace prolonged deferrals and contact the recipient through another appropriate channel only if necessary. Share the redacted reply and times, without including the original confidential message or account passwords.
Work through these checks in order
- Keep the enhanced status and text that follow 450. A quota-related reply and a policy deferral require different owners and should not be treated as the same mailbox problem.
- Inspect the sender’s retry history and the receiving provider’s status. If the text names a full recipient mailbox, the recipient administrator may need to act.
- Allow the provider’s normal queue handling while the identified issue is corrected. Check the final result before sending another copy or marking a recipient permanently invalid.
Which tool can help?
SMTP error lookup · MX record checker
DNS tools show one resolver’s public answers. Record explainers do not authenticate a message, and calculators do not monitor a server. Use the evidence alongside your provider’s logs.