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550 5.3.4: Microsoft 365 message too big for route

550 5.3.4Message size exceeds fixed maximum sizePermanent: retrying will not helpFrom Microsoft 365 and Outlook.com

Example
550 5.3.4 ROUTING.SizeLimit; message size exceeds fixed maximum size for route

What it means

RFC 3463 calls X.3.4 message too big for system. In Exchange Online the ROUTING.SizeLimit variant means some size limit along the route was smaller than the message: the sender’s mailbox send limit, the recipient’s receive limit, a transport rule, or the organisation maximum.

Remember that the size checked is the encoded size. Base64 encoding grows attachments by roughly a third, so a 25 MB file becomes a message of about 33 MB on the wire. That is why a file that looks small enough on disk still bounces.

This is different from a remote server answering 552 during SMTP because its own limit is lower; that rejection comes from the other system and carries its own text, while ROUTING.SizeLimit is Microsoft’s own routing check.

Likely causes

  • An attachment that is under the limit on disk but over it after MIME encoding.
  • A recipient mailbox or distribution group with a lower MaxReceiveSize than the tenant default.
  • A sender mailbox with a reduced MaxSendSize set by an admin.
  • A connector or transport rule that applies its own size cap to the route.

How to fix it

  1. Sender: share large files through a OneDrive or SharePoint link instead of attaching them.
  2. Sender: compress or split the attachment so the encoded message stays under the limit.
  3. Admin: check MaxSendSize on the sender and MaxReceiveSize on the recipient with Get-Mailbox, then raise them (the Exchange Online ceiling is 150 MB).
  4. Admin: look for transport rules and connectors that apply a lower size condition.

Checks that apply

  • SMTP Diagnostics For external routes, shows the SIZE value the receiving server advertises in EHLO, so you can tell whether their limit or yours is lower.
  • MX Lookup Identifies which server receives mail for the domain when the size limit might be on the far side.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

Why did a 20 MB attachment fail with a 25 MB limit?
Encoding adds about 33 percent, plus headers and body. The message on the wire is larger than the file.
Can I raise the limit for everyone?
Admins can raise mailbox limits up to 150 MB, but the receiving side may still reject large mail, so links are more reliable.

Microsoft 365 delist portal Outlook.com SNDS

All 191 bounce codes