432 4.3.2: recipient mailbox is receiving too fast
Exchange Online is slowing delivery to one mailbox because it is receiving too many messages at once; the sending server should retry later.
- Microsoft
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 432 4.3.2
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
432 4.3.2 STOREDRV.Deliver; recipient thread limit exceeded
What it means
This is a temporary deferral, issued by Exchange Online when a single recipient mailbox takes in messages faster than the service allows. Microsoft throttles deliveries to that one mailbox so that its processing does not slow down the other mailboxes stored on the same database. The message is not rejected: a normal sending server keeps it in its queue and tries again, and it is delivered once the burst passes. Nothing is wrong with the sender or the address. If it keeps happening, the mailbox is the target of a flood, often from an automated system or a loop.
Common causes
- Many messages arrived for the same mailbox within a short time, for example from an alerting or notification system.
- A mail loop or a misbehaving application is sending repeated copies to one recipient.
How to fix it
- 01Let the sending server retry: the deferral clears on its own once the volume to that mailbox drops.
- 02If you control the sending system, spread its messages to this recipient over a longer period.
- 03If you are the recipient's admin, find the source of the burst in message trace and stop any loop or noisy application.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
Related codes
- 5.2.121Too many messages to one recipient this hour.The sender has gone over the number of messages Exchange Online lets one sender deliver to the same recipient within an hour.
- 5.2.122Recipient has received too many messages this hour.The Microsoft 365 recipient has hit the hourly limit on messages it can receive from all senders combined, so new mail is refused for now.
- 4.3.2Mailbox folder holds too many items.Exchange Online cannot deliver to this mailbox because one of its folders has reached the limit of one million items.
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce