Bounce Codes
450 4.2.0 Temporary Failure Mailbox temporarily unavailable
The mailbox exists but can't accept mail right now — commonly a full inbox or a brief server-side issue specific to that mailbox.
Common causes
- Recipient's mailbox has exceeded its storage quota.
- Recipient's mail server is undergoing brief maintenance.
What to do
Retry later. If this persists across multiple attempts over days, treat similarly to a soft-bounce pattern and consider the address likely abandoned.
FAQ
Should a verification tool mark this address as invalid?
No — this is a temporary condition, not a confirmation the mailbox doesn't exist. A well-built verifier retries before reporting a final verdict rather than treating a single 450 as permanent.
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