SMTP error 5.2.3 Message length exceeds administrative limit

What SMTP error 5.2.3 means

Your message is bigger than this specific mailbox is allowed to receive. It comes back as 552 with wording about the size exceeding a limit, and the key word is administrative: somebody has capped that recipient below what the platform as a whole would accept. Another recipient at the same organisation might take the identical message without complaint, and that is what separates it from a system-wide size rejection.

Mailbox status
Sent by The receiving system, applying a limit set for that particular recipient
Occasional

What SMTP error 5.2.3 Message length exceeds administrative limit actually means

Mail platforms enforce size in layers: a ceiling for the whole service, limits on individual connectors and gateways, and limits attached to particular mailboxes or applied by transport rules to particular groups. This code reports the last of those. In Exchange Online it maps to a mailbox receive limit set with Set-Mailbox, or to a transport rule capping what certain recipients may be sent. Do the arithmetic before arguing about the number: attachments are base64 encoded for transport, which inflates them by roughly a third, so a twenty-five megabyte file arrives as around thirty-three megabytes and a twenty-five megabyte limit will not pass it.

A per-mailbox administrative message length limit has been exceeded. This status code should be used when the per-mailbox message length limit is less than the general system limit. This code should be used as a permanent failure.
— RFC 3463

How 5.2.3 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 5.2.3 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 5.2.3 on a business phone system

A receive limit set on that individual mailboxOften applied years ago to a mailbox that was struggling, or to a shared mailbox somebody wanted to keep tidy, and then forgotten. It survives migrations and plan changes because nobody thinks to look for it.
A transport rule capping message size for a groupRules rejecting anything over a set size for a department or a mailbox used by an automated process. They are invisible from the mailbox properties, so they are missed when someone checks the obvious place and finds a default.
Base64 encoding pushing the message past a limit it appears to be underThe one that generates arguments. Three files of eight megabytes each are not twenty-four megabytes on the wire but closer to thirty-two, plus body and headers. The limit is applied to the encoded size, not the file on your disk.
A tighter limit on a filtering gateway in front of the mailboxMimecast, Barracuda and Proofpoint all apply their own size policies, set per recipient or per policy group. The gateway rejects before Exchange Online is consulted, which is why the tenant settings look fine.
A legacy limit inherited from an on-premise Exchange serverOrganisations that migrated from Exchange 2010 or 2013 often carried across a ten megabyte convention that suited the old hardware. In a hybrid setup that value can still apply to routes traversing the old server.

How to fix SMTP error 5.2.3

Read the limit out of the bounceMost systems state the maximum they will accept in the rejection text. That number tells you whether you face a per-mailbox restriction or something close to the platform ceiling, and it saves guessing.
Check the recipient's own limit rather than the tenant defaultGet-Mailbox with the MaxReceiveSize property shows what applies to that mailbox specifically. If it differs from the organisation default, you have found the cause without touching anything else.
Search the transport rules for size conditionsFilter the mail flow rules for any condition on message size and check whether the failing recipient falls into scope. Rules built around group membership catch people out, because the mailbox itself looks unrestricted.
Send a link instead of an attachmentUpload to SharePoint or OneDrive and share a link. It sidesteps every size limit in the chain, gives you a record of who opened the file, and lets you withdraw access afterwards.
Raise the limit deliberately rather than to the maximumIf the cap is genuinely wrong, set a considered value and apply the same number across mailboxes and any gateway. Raising one mailbox to the ceiling while the gateway in front stays low just moves the rejection somewhere less obvious.

Fixing the underlying problem

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

Questions about SMTP error 5.2.3

My attachment is under the stated limit, so why was it rejected?

Because the limit applies to the encoded message, not the file on your disk. Attachments grow by roughly a third when base64 encoded, and the body, headers and signature add more. As a rule of thumb, keep attachments to about three quarters of whatever limit you have been given.

Why does it work for one person at that company and not another?

That is the signature of a per-recipient limit rather than a system one. Somebody has capped that particular mailbox, or a rule applies to a group they belong to. Their IT team can see it in seconds, and nothing you change at your end will get around it.

Should we just raise the limit for everyone?

Only up to a point. Large attachments move slowly, bloat mailboxes and count against quotas at both ends, and there is a hard platform ceiling you cannot exceed anyway. Sharing a link is faster, leaves no copy in anyone's mailbox and can be revoked.

Related SMTP status codes

Source. SMTP error 5.2.3 Message length exceeds administrative limit 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