Outlook attachment size limit

Outlook.com limits an email to 25 MB, attachments included. Microsoft 365 work and school accounts default to 35 MB sent and 36 MB received. A default on-premises Exchange server stops at 10 MB. In every case, the file you can actually attach is about a quarter smaller than the number, because of how email encodes attachments. The details, and what to do once you are over, follow.

The limits, by kind of account

Two details catch people out. The limit is per message, not per file: two 15 MB attachments will not go together under a 25 MB cap. And in Outlook on the web, Microsoft 365 lets you send only about 25 % less than the configured limit, to leave room for encoding — which is where the ~26 MB figure at the 35 MB default comes from.

20 MB or 25 MB? Microsoft's own pages disagree

If you have seen “20 MB” quoted for Outlook, you are not misremembering. Microsoft's Outlook.com help page gives 25 MB for an email. Its general page on sending large files with Outlook gives 20 MB for “internet email accounts such as Outlook.com or Gmail”, and 10 MB for business Exchange accounts — a figure that matches on-premises Exchange servers but not the 35 MB default of Microsoft 365.

The practical answer does not depend on which page is right. Keep attachments under about 14 MB and they pass under either reading, on any Outlook account, and on most other providers too.

The smaller number hiding behind each limit

Email cannot carry raw binary data, so attachments are converted to text with base64 encoding, which inflates them by about 33 %. The size check runs on the encoded message, not on your file.

And then there is the other server

A message Outlook happily sends can still bounce later. The recipient's mail server enforces its own maximum, and the lower of the two limits wins. Gmail, for instance, sends up to 25 MB, and many corporate servers are configured at 10 MB. Sending 30 MB from one Microsoft 365 colleague to another may work, and the same message to a client outside your organisation may not — and you learn that from a delivery failure notice, sometimes hours later.

What Outlook suggests, and what it costs

Attach something over the limit and Outlook offers to upload it to OneDrive and insert a link instead. Microsoft allows linked files up to 2 GB, and links do not count towards the email size limit. For a holiday video it is a perfectly good answer. For anything you would rather not hand over, know what happens:

• A full copy of the file is stored on Microsoft's servers until you delete it.
• Microsoft can read it. The file is encrypted at rest, but Microsoft holds the keys — that is encryption in transit and at rest, not end-to-end encryption.
• The link is the credential. Depending on how it is shared, it can be forwarded or pasted somewhere it was never meant to go.
• It uses your OneDrive storage, which on a free Microsoft account is 5 GB.

The private way to send a bigger file

When the file is sensitive — a contract, a scan, financial records, client work — the goal is to get past the limit without creating a copy anyone else can read. A peer-to-peer transfer does that: the file is encrypted in your browser with AES-256-GCM and streams straight to the recipient's browser. Nothing is uploaded, so there is no stored copy to leak or forget about.

The decryption key lives in the link's URL fragment, after the #, which browsers never send to a server, and a separate PIN gates the transfer. With SaferDrop you sign in to send, and a transfer carries up to 100 MB on the free plan and up to 2 GB on Pro. The recipient needs no account and installs nothing. The trade-off: you both need to be online while it streams, and the link expires within an hour.

You can keep using Outlook for the message itself. Email the link, send the PIN by text or phone, and the mail server only carries a URL that is useless on its own — the pattern described in how to send files securely by email.

Every provider's limit side by side is in file too large to email. If it is Gmail you are dealing with, see the Gmail attachment size limit.

Sources, checked on 18 October 2026: Unable to attach files in Outlook.com, Send large files with Outlook, Exchange Online limits. Administrators can change any of these limits for their organisation.

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