Temporary — will retry
SMTP 450 4.2.1
mailbox temporarily unavailable
SMTP 450 4.2.1 means the mailbox exists but cannot accept mail right now — commonly because it is disabled, over quota, or the server is rate-limiting you. It is a temporary failure, so the sending server will retry, and legitimate mail usually arrives on a later attempt.
How it appears
450 4.2.1 The user you are trying to contact is receiving mail too quickly
450 4.2.1 <user@example.com>: Recipient address rejected: mailbox temporarily unavailable
450 4.2.1 Mailbox disabled, not accepting messages
What causes it
- The mailbox is temporarily disabled or suspended by an administrator.
- The recipient server is rate-limiting your sending IP for volume rather than content.
- The mailbox is over quota and the server prefers a soft rejection to a hard one.
- A per-recipient throttle has been hit, common on large corporate tenants.
How to fix it
In order — try the first one first.
- Let the retry happen — this is not a verdict on the address.
- If a whole domain returns it, slow your send rate to that domain rather than retrying harder.
- If it persists for days on a single address, treat the mailbox as effectively dead and suppress it.
- For verification, this is an inconclusive result and is correctly reported as risky, not invalid.
Common questions
- What does SMTP 450 4.2.1 mean?
- SMTP 450 4.2.1 means the mailbox exists but cannot accept mail right now — commonly because it is disabled, over quota, or the server is rate-limiting you. It is a temporary failure, so the sending server will retry, and legitimate mail usually arrives on a later attempt.
- Should I retry a 450 4.2.1 error?
- Yes, and your sending server does it automatically. 450 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.
- How do I fix SMTP 450 4.2.1?
- Let the retry happen — this is not a verdict on the address.
Last reviewed 2026-09-11