4.3.2: mailbox folder holds too many items

What it means

Despite the word busy and the 4.3.2 code, which normally signals a temporary condition, Microsoft documents this reply for a mailbox where a single folder holds one million items, the Exchange Online limit per folder. The store cannot add another item, so delivery fails until the folder is cleared, and retrying does not help while it stays full. Microsoft prints it with a 554 reply, a permanent class, which is why the reply and the code disagree. The fix belongs to the mailbox owner or their admin.

Common causes

• A folder in the recipient mailbox, often the Inbox or a folder fed by a rule, has reached one million items.
• An automated feed or rule has been filing messages into the same folder for years without cleanup.

How to fix it

1. 01Tell the recipient or their admin, quoting the bounce: only they can clear the folder.
2. 02Admins: run Get-MailboxFolderStatistics for the mailbox to find the folder that has reached the limit.
3. 03Move items out of that folder into other folders or the archive mailbox, then ask the sender to resend.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• "554 4.3.2 mailbox busy" when sending emails to a mailbox in Exchange Onlinelearn.microsoft.com

Further reading

• The standard meaning of 5.3.2: server not accepting mail