4.3.2
Enhanced Status Code 4.3.2: System Not Accepting Messages
Temporary failure
Medium severity
System
RFC 3463
What it means
Enhanced Status Code 4.3.2 means “System Not Accepting Messages.” The mail system is temporarily not accepting new messages. This could be due to maintenance, system overload, or an administrative decision to pause incoming mail. Retry later.
At a glance
Code4.3.2
Bounce typeSoft (temporary)
SeverityMedium
CategorySystem
What to doQueued and retried automatically
StandardRFC 3463
451 4.3.2 The maximum number of concurrent server connections has exceeded a limit, closing transmission channelWhat does 4.3.2 mean?
Enhanced status code 4.3.2 means the receiving mail system has temporarily stopped accepting new messages. This is different from a full storage condition (4.3.1) - the server may have plenty of capacity but is intentionally not accepting mail, perhaps for maintenance, upgrades, or load management.
This is a transient condition that should resolve automatically. Your MTA should retry delivery over the next several hours.
How 4.3.2 plays out
Your server attempts delivery
The recipient defers with a temporary
4.3.2 replyYour server queues the message and retries on a back-off schedule
It delivers on a later attempt, or becomes a hard bounce if it keeps failing
Where 4.3.2 sits: soft vs hard bounce
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| SMTP class | 4xx | 5xx |
| What to do | Let it retry | Suppress the address |
| Recoverable? | Often | No |
| 4.3.2 is | ✓ this code |
Common causes of 4.3.2
- Server undergoing maintenance or upgrades
- System overloaded and temporarily rejecting new mail
- Administrative decision to pause incoming mail
- Anti-spam system temporarily blocking all incoming messages
How to fix 4.3.2
- Wait for automatic retry - this is typically resolved within hours
- If persistent, contact the recipient organization postmaster
Frequently asked questions
What does bounce code 4.3.2 "System not accepting messages" mean?
Bounce code 4.3.2 means the recipient's mail system is temporarily unable to accept incoming messages due to conditions such as an imminent shutdown, excessive load, or scheduled maintenance. This is a temporary error; the server is operational but has intentionally stopped accepting mail until the condition is resolved. Your sending server will automatically retry delivery.
How long does a 4.3.2 error last?
A 4.3.2 error typically resolves within a few hours once the underlying condition (maintenance, overload, or resource shortage) is addressed. Most email systems automatically retry delivery for 3-5 days, so manual resending is usually unnecessary. If the error persists beyond 24 hours, it may indicate a more serious server problem, and you should contact the recipient's mail administrator or reach the recipient through an alternative channel.
How do I fix a 4.3.2 "Service not available" error?
As a sender, wait for the recipient's server to resume normal operations; your mail system will automatically retry. Do not manually resend immediately, as this creates duplicate messages when the server recovers. If you are the server administrator receiving this error, check that your receive connectors are properly configured, restart the Exchange Transport Service, verify that your mail security appliance IP addresses are whitelisted, and ensure the server has adequate system resources (CPU, memory, disk space).
Related bounce codes
Sources & further reading
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026
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