The default Outlook attachment size limit for a Microsoft 365 mailbox is 35 MB for sending and 36 MB for receiving, and that size covers the whole message, attachments included. An administrator can raise it to as much as 150 MB, but on Outlook on the web the ceiling is 112 MB and on the iPhone and Android apps it's 33 MB.
Here's the catch. The 150 MB figure you'll see quoted is a ceiling an admin has to set, not what you get out of the box. And even if your side allows it, the email only arrives if the other side's server accepts that size too.
The limits at a glance
| Limit | Size, as of October 2026 |
|---|---|
| Default sending limit | 35 MB |
| Default receiving limit | 36 MB |
| Custom limit an admin can set | 1 MB to 150 MB |
| Outlook for Windows and Outlook for Mac | 150 MB maximum |
| Outlook on the web | 112 MB maximum |
| Outlook for iOS and Android | 33 MB maximum |
| Mailbox migration | 150 MB maximum |
| Attachments per message | 250 |
| Subject line | 255 characters |
The app figures are maximum values. Whatever limit your admin has set still applies, so if the tenant is on the 35 MB default, that's your real limit in every app.
Message size, not file size
The message size limit includes the attachments, so what matters is the total of everything in the email, not each file on its own. Three 15 MB files make a 45 MB message, which is over the default limit even though no single file is.
There's a separate limit per individual file, too: 150 MB in Outlook and 112 MB in Outlook on the web, again as maximum values. Admins can also create transport rules to limit the size of any single attachment, so your organisation may have a lower per-file cap than the message limit.
Why the other side matters
Some email clients impose lower limits, or lower per-attachment limits, than the Exchange Online message size limit. So you can send a 60 MB email from a tenant that allows it and still have it fail because the person you're sending to can't receive it.
When a message is larger than the limit, it isn't delivered and you get a non-delivery report, often called an NDR or a bounce. That report is your signal to shrink the file or switch to a link.
Big groups, encryption and headers
- A message sent to a distribution group of 5,000 or more people has its own cap: 25 MB for groups of 5,000 to 99,999 members, and 5 MB for groups of 100,000 members.
- Encrypted messages are limited to 100 MB with Microsoft Purview Message Encryption, and 25 MB with the older Office 365 Message Encryption, which is deprecated.
- The header size limit is 256 KB in total and 32 KB per header. Over that, Exchange Online rejects the message with the code 552 5.3.4.
Most small businesses won't hit the group or header limits, but the encryption one matters if you send contracts or financial documents securely.
The OneDrive route for anything bigger
For a file that's too big for email, share it from OneDrive instead. The OneDrive upload limit is 250 GB per file, which is far beyond any email limit, and the person gets a link rather than a heavy attachment. If you've not shared from OneDrive before, here's how to share a OneDrive folder.
What to do when an attachment won't send
- Add up the size of the whole message, including every attachment, rather than checking each file.
- If it's under 35 MB it should send. If it's over, ask your admin whether the limit has been raised.
- Check which app you're using. Outlook on the web tops out at 112 MB and the mobile apps at 33 MB.
- For anything bigger, share a OneDrive link instead of attaching the file.
- If you get a non-delivery report, shrink the file or switch to a link and send again.
If what you're attaching is another email rather than a file, my guide on attaching an email in Outlook walks through it.
My take on Outlook attachment limits
I'd treat 35 MB as the real number for anything going outside your business, even if your admin has raised your own limit, because you can't control what the other side accepts. I'd only ask for a higher limit if your team swaps large files internally all day. For everything else, a OneDrive link is quicker to send, doesn't clog anyone's mailbox, and won't bounce.
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Frequently asked questions
What is the maximum attachment size in Outlook?
The default is 35 MB for sending, as of October 2026. An admin can raise it to as much as 150 MB, with 112 MB the maximum on Outlook on the web and 33 MB on the mobile apps.
Does the Outlook limit apply per file or per email?
The message size limit applies to the whole email, attachments included. There's a separate per-file maximum of 150 MB in Outlook and 112 MB on the web.
How many attachments can I add to one Outlook email?
Up to 250 file attachments per message.
What happens if my Outlook email is too large?
It isn't delivered, and you receive a non-delivery report.
How do I send a file that is too big for Outlook?
Upload it to OneDrive, which accepts files up to 250 GB, and share a link instead.
What is the size limit for encrypted emails in Outlook?
100 MB with Microsoft Purview Message Encryption, and 25 MB with the older, deprecated Office 365 Message Encryption.
Is the limit lower when emailing a large distribution group?
Yes. A message to a group of 5,000 to 99,999 members is capped at 25 MB, and to a group of 100,000 members at 5 MB.
Can my admin limit the size of a single attachment?
Yes. Administrators can create transport rules to limit the size of any single attachment.
