Guide
5.3.4 “Message size exceeds fixed maximum message size”
The message is too large, says Microsoft: the limit can be the sender’s mail system or the recipient’s. Resend it without the attachment, with a link: up to 5 GB here, without an account.
What the error says
| Who refused | The sending or the receiving mail system |
|---|---|
| Microsoft 365, by default | 35 MB to send, 36 MB to receive; the administrator can set 1 to 150 MB |
| What Microsoft suggests | Send again without attachments, or configure a larger limit |
Sources: Microsoft Learn, Outlook help — October 2026.
How to send bigger
A TransferThing link
Drop the file here, resend the message with the link instead of the attachment. Up to 5 GB, without an account for you or the recipient.
A OneDrive link
From Outlook, share the file from OneDrive, up to 2 GB. It uses your OneDrive storage.
A larger limit
Only an administrator can raise it, up to 150 MB on Microsoft 365, and the other side’s server may keep its own.
A free account to follow your transfers.
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Frequently asked questions
What does 5.3.4 “Message size exceeds fixed maximum message size” mean?
The message is too large, according to Microsoft: the sending or the receiving mail system refused it, and you get a non-delivery report.
What is the size limit on Microsoft 365?
35 MB to send and 36 MB to receive by default, attachments included; an administrator can set a limit between 1 MB and 150 MB.
Can I fix it myself?
Not the limit: only an administrator can change it, and it may be the recipient’s. What you can change is the message: send it again without the attachment.
How do I send the file anyway?
Drop it on TransferThing and resend the message with the link: up to 5 GB, without an account.
Does the recipient need an account?
Not for a TransferThing link: they download in their browser, without an account.
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