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.
Should you remove the address?
Keep the address; fix the sending side
Keep it. The address is real and active; Gmail is limiting how fast it receives mail.
Soft bounce. Your sending server retries on its own. Investigate only if it keeps failing.
How it appears
450 4.2.1 The user you are trying to contact is receiving mail too quickly
450 4.2.1 <[email protected]>: 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.
Who sends it
Gmail and Google Workspace. It shows up most when many messages go to one address in a short time, such as alerts, form notifications or a loop.
Gmail
Reply text The user you are trying to contact is receiving email too quickly. Please resend your message at a later time.
Gmail also has a version that says the rate "prevents additional messages from being delivered".
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.
- Is 450 4.2.1 a hard bounce or a soft bounce?
- 450 4.2.1 is a soft bounce. It is temporary, and your sending server retries it automatically. Your sending server retries on its own. Investigate only if it keeps failing.
- Should I remove the address after a 450 4.2.1 bounce?
- Keep it. The address is real and active; Gmail is limiting how fast it receives mail.
- Will verifying my list prevent 450 4.2.1?
- Only partly. Temporary deferrals are not about the address, but providers defer senders whose mail looks risky, and a high rate of hard bounces is one of the signals they use.
Clean the list before the next send.
A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.
- A real SMTP mailbox check on every row, before anything is sent
- Dead and mistyped addresses split out, so they never reach your sending tool
- Catch-all, disposable and role addresses flagged alongside
Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.
Other 450 replies
Sources
- Source: Google Workspace Admin Help: Gmail SMTP errors and codes
- Source: RFC 3463: Enhanced Mail System Status Codes
Provider wording is quoted only where the provider publishes it.
Last reviewed 2026-10-22