451 4.7.500 Server busy, please try again later
Microsoft is throttling your IP. Any enhanced code from 4.7.500 to 4.7.699 is a temporary deferral of this kind. The message stays in your queue and a normal retry schedule delivers it; the wording "Server busy" is about your allowance, not Microsoft's capacity.
What the server replies
451 4.7.500 Server busy. Please try again later from [203.0.113.10]. (S3214) [DM6NAM11FT042.eop-nam11.prod.protection.outlook.com]
- Sent by
- Microsoft 365
- Type
- TemporaryRetry helps
- About
- Rate limits and quotas
- Enhanced code
- 4.7.500
Who sends it
Exchange Online Protection, at RCPT TO or DATA, when an IP exceeds the rate Microsoft is willing to accept from it. New IPs have low allowances; established senders are throttled after a spike or a run of complaints.
Why it happens
- A new or recently quiet IP sending more than Microsoft expects of it.
- Too many simultaneous connections from one host.
- A campaign with a high rate of unknown recipients or complaints in the previous hours.
- A shared IP where another sender's volume consumed the allowance.
How to fix it
Retry, slower
Leave the messages queued and cut the per-connection message count and concurrency toward outlook.com and protection.outlook.com destinations. Most MTAs let you set this per destination domain.
Ramp volume gradually
For a new IP, increase daily volume to Microsoft recipients in steps over two to three weeks instead of sending the whole list on day one.
Review the list
Deferrals that come with 5.4.1 bounces mean many recipients do not exist. Clean the list; Microsoft reads unknown-recipient attempts as a spam signal.
Check SNDS
Microsoft's Smart Network Data Services shows whether the IP is marked yellow or red and why.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
Related codes
- 550 5.7.708 Service unavailable, access denied, traffic not accepted from this IPMicrosoft is declining all traffic from the sending IP, a softer cousin of the 5.7.606 ban.
- 421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP addressGmail is throttling the sending IP.
- 421 4.7.0 [TSS04] Messages from IP temporarily deferred due to unexpected volume or user complaintsYahoo is slowing your IP down.
- 450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being deliveredThe limit here belongs to the recipient, not to you.
Sources
Fewer bounces to look up.
Faivelo queues temporary failures and retries them on a schedule receivers accept, then tells your application once, in plain words, if a message finally cannot be delivered.
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Questions people ask
Other free tools
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- Email address validatorCheck whether an address is well formed, whether its domain accepts mail, and whether it is disposable or a role account.