How to fix Thunderbird temporary server errors 421, 451 and 452
These are temporary (4xx) SMTP errors: the server is overloaded, throttling you, or you have hit a sending limit. They usually clear by themselves. Jump to your situation below or work through the methods in order.
By Neeraj Singh ~5 min Updated Jun 2026 89% found this helpful
Error message
421 Service not available / 451 Local error in processing / 452 Too many recipients or insufficient storage.
Summary
SMTP codes in the 4xx range are temporary, the server is telling you to try again later rather than rejecting the message outright. 421 means the service is not available right now (overloaded or shutting down a connection), 451 means a local error in processing on the server, and 452 means insufficient system storage or, very commonly, too many recipients in one message. The usual real-world cause is the provider rate limiting you: too many messages in a short time, too many recipients, or hitting a daily sending cap. Because these are transient, the first fix is simply to wait an hour or two and resend. Reducing the number of recipients per message, slowing your sending, and checking the provider's documented limits resolve the rest. Greylisting servers also return a temporary error on first contact and accept the retry.
What this error means
Every SMTP reply has a numeric code, and the first digit tells you the category. A 4 means temporary: the server could not take the message now but invites you to try again. This is different from a 5xx permanent rejection, which will not succeed on retry without a change.
For 421, 451 and 452 the cause is usually load or limits rather than anything wrong with your message. Providers throttle to fight spam, so sending a lot quickly, adding many recipients, or passing a daily cap all trigger a temporary error. Waiting and easing off is the correct response.
Common causes
The server is temporarily overloaded or unavailable (421).
A temporary local processing error on the server (451).
Too many recipients in a single message (452).
You exceeded the provider's daily sending limit.
You are sending too many messages too quickly.
A greylisting server is deferring the first attempt.
Expert insight
“The number to watch is the first digit, and a 4 means not now, try again. So with 421, 451 and 452 I do not start changing settings, I wait. Most of the time the queue clears or the rate limit resets and the next send works. If it keeps happening, it is usually too many recipients in one go or bumping the daily cap, so I split the recipients and slow down. Greylisting does this too, it bounces you once on purpose and accepts the retry.”
Manager, Tech Support & Operations · 19+ years fixing Windows and system errors
✓ How to fix it
Method 1
Wait and resend
1Because these are temporary errors, the simplest fix is to wait an hour or two and send again.
2Thunderbird will often retry queued messages on its own.
3Many cases clear with no other change.
Method 2
Reduce recipients per message
1A 452 frequently means too many recipients in one message.
2Split a large recipient list into smaller batches.
3Send the batches a little apart.
Method 3
Check the provider's sending limits
1Most providers cap how many messages or recipients you can send per day.
2Check your provider's documented limits and stay under them.
3If you hit a daily cap, wait until it resets.
Method 4
Slow your sending rate
1Sending many messages in quick succession can trigger throttling.
2Space your sending out rather than blasting all at once.
3This avoids the rate limit that causes the temporary error.
Method 5
Confirm it clears
1After waiting or easing off, send a single test message.
2If it goes through, the temporary condition has cleared.
A 4xx code means temporary, so the first and best response is to wait and resend rather than change settings. If 452 keeps appearing, the usual cause is too many recipients in one message or hitting a daily cap, so split the list and slow down. Greylisting also returns a temporary error on purpose and accepts the automatic retry.
Frequently asked questions
What do SMTP errors 421, 451 and 452 mean?
They are temporary (4xx) errors. 421 means the service is unavailable, 451 a local processing error, and 452 insufficient storage or too many recipients. The server is asking you to try again later.
Do I need to change any settings?
Usually not. These are temporary conditions, so waiting an hour or two and resending is the main fix. Settings changes are only needed if you are hitting recipient or rate limits.
Why do I get 452 specifically?
452 most often means too many recipients in a single message, or the server is low on storage. Split a large recipient list into smaller batches and resend.
Could I be hitting a sending limit?
Yes. Providers cap daily messages and recipients to fight spam. If you send a lot, you can hit the cap and get a temporary error until it resets. Check your provider's limits.
What is greylisting?
A spam-fighting technique where the server temporarily rejects the first delivery attempt and accepts the retry. It produces a temporary error that clears automatically on the next attempt.
How long should I wait?
An hour or two is usually enough for an overload or rate limit to clear. A daily cap resets after 24 hours. Thunderbird often retries queued mail automatically.
Still not working?
If a temporary error never clears even after waiting, the server may be persistently deferring you over reputation or authentication. Check that your sending domain has valid SPF and DKIM, and contact the provider if your messages are being throttled as suspected spam. You can also submit your error to us for a tailored fix.
