SMTP error 5.2.2 Mailbox full

What SMTP error 5.2.2 means

The recipient's mailbox has no room left. It arrives as 552 5.2.2 worded as mailbox full or over quota, and it is entirely a problem at the receiving end — nothing about your message caused it and resending will not help. In Microsoft 365 it means the mailbox has crossed its prohibit-send-receive threshold, fifty gigabytes on most business plans and a hundred on the higher ones. Some servers report the same condition as 4.2.2 and keep retrying.

Mailbox status
Sent by The recipient's mail platform, once the mailbox has hit its quota
Common

What SMTP error 5.2.2 Mailbox full actually means

Exchange Online applies three thresholds on the way to a mailbox ceiling: a warning that emails the user, a prohibit-send point where they can still receive but cannot send, and a prohibit-send-receive point where inbound delivery stops too. This code belongs to the last of those. Note the tension in the specification itself, which describes the condition as one that ought to be treated as a persistent transient failure while IANA lists the permanent 552 alongside it. Hence the variation in the wild: some systems bounce at once, others hold the message under a 4.2.2 and retry on the assumption the recipient will delete something.

The mailbox is full because the user has exceeded a per-mailbox administrative quota or physical capacity. The general semantics implies that the recipient can delete messages to make more space available. This code should be used as a persistent transient failure.
— RFC 3463

How 5.2.2 appears in a bounce

A server reporting this condition sends it alongside a three-digit reply code, and a bounce prints the two together. The registry lists 552 5.2.2 as the pairing for this status.

The leading 5 is the server's verdict rather than part of the code's identity: it marks this as a permanent failure, which means the sending server has given up and returned the message.

What causes SMTP error 5.2.2 on a business phone system

The mailbox has simply grown into its quotaYears of attachments, a Sent Items folder nobody has pruned and a habit of never deleting anything. Fifty gigabytes sounds enormous until a decade of PDFs and photographs has accumulated in it.
Online archiving has never been switched onBusiness Premium and the enterprise plans include an archive mailbox with separate space, and a default policy moving items over two years old into it. Where it was never enabled, everything stays in the primary mailbox and counts against the quota that matters.
The Recoverable Items folder has filled upDeleted items go to Recoverable Items, which has its own quota. Put a mailbox on litigation hold or a retention policy and nothing is ever purged, so that folder grows quietly and consumes the mailbox from the inside while the visible folders look reasonable.
A shared mailbox that has outgrown its unlicensed allowanceA shared mailbox is free up to fifty gigabytes and needs a licence beyond that. Busy team addresses handling invoices or job applications reach that point faster than anyone expects, and then stop receiving.
A large one-off delivery pushing the mailbox over the lineA mailbox sitting just under its threshold takes a report with a substantial attachment and crosses it. The next message bounces though nothing structural has changed, which is why these appear to come out of nowhere.

How to fix SMTP error 5.2.2

Tell the recipient, then let them clear spaceIf the mailbox is not yours, only they can act. Deleting large items and emptying Deleted Items is the quick route, and Outlook's Mailbox Cleanup tool sorts by size so the worst offenders surface first.
Check the actual usage rather than the user's estimateMailbox usage in the Exchange admin centre shows current size against quota. Get-MailboxStatistics reports the same figure and, importantly, the size of the Recoverable Items folder, which is where the space has gone in the puzzling cases.
Enable the online archive and let the default policy runTurning on archiving and applying the default retention policy moves items older than two years into archive space over the following days. The least disruptive fix, because nothing is deleted, and it usually recovers a great deal of room.
Raise the quota or move the user to a plan carrying moreCompare the plans before buying anything extra. The higher enterprise plans include a hundred gigabytes and unlimited auto-expanding archiving, often cheaper than managing a permanently full fifty gigabyte mailbox.
Set up an alert so it does not happen againWarning thresholds exist for exactly this, and users ignore the notification emails. Review mailbox sizes monthly, or alert on anything crossing eighty per cent, so the first sign of trouble is not a bounced customer message.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SMTP error 5.2.2

Should I keep resending, or will it get through on its own?

It depends on the class digit. A 4.2.2 means the sending server has kept the message and will retry, so leave it and it may well arrive once space is freed. A 5.2.2 means it has already been returned, so send it again once the recipient confirms they have made room.

Our mailbox says it is nowhere near full, so why are we bouncing?

Look at the Recoverable Items folder. It has its own quota, and under a litigation hold or retention policy nothing is ever purged from it, so it can block the mailbox while the folders the user sees look healthy. Get-MailboxStatistics reports it, and auto-expanding archiving is the usual remedy.

How long will Microsoft 365 keep trying before it gives up?

For a message it has queued rather than bounced, Exchange Online retries with increasing gaps and returns it after around twenty-four to forty-eight hours. The sender normally gets a delayed-delivery warning part-way through, which is a prompt to ring the recipient rather than wait.

Is buying more storage the right answer?

Sometimes, but check what you already pay for. Most business plans include an archive mailbox with separate space at no extra cost, and it is frequently switched off. Enabling it and letting the default two-year policy run recovers more room on a long-lived mailbox than any quota increase you would buy.

Related SMTP status codes

Source. SMTP error 5.2.2 Mailbox full is defined in RFC 3463 and registered in the IANA SMTP Enhanced Status Codes registry. The causes and fixes above are drawn from our own experience supporting UK business email systems.

All Email delivery & SMTP codes