Outlook Attachment Limit in 2026: 20 MB, 25 MB, or 150 MB?

Outlook is the email client where “what's the attachment limit?” has four different answers. You can check three help pages and come away more confused than when you started.

The short version: free Outlook.com is 25 MB. Desktop Outlook with a non-business account is 20 MB. Microsoft 365 business mailboxes default to 35 MB and can go up to 150 MB if an admin raises it. The new Outlook for Windows silently uploads anything bigger to OneDrive.

Which one applies to you depends on your account type, not the app you're looking at. Here's how to figure out your real limit and the easiest way around it.

What is the Outlook attachment size limit?

The 25 MB figure for free Outlook.com is the limit on the total email size. Attachments, body, and inline images combined. The 20 MB on desktop is a Microsoft-imposed client default, and the 150 MB ceiling on business mailboxes is the hard cap in Exchange Online's service limits, not a soft suggestion.

Why your file fails even when it's under the limit

This is where people get stuck. You have a 19 MB file, the limit is 20 MB, and the email still bounces.

The reason is the same as in Gmail: email has to repackage your file into a text-based format (MIME, specifically base64 encoding) before it can ride along with the message. That repackaging adds about a third to the size. A 19 MB file becomes roughly 26 MB by the time Outlook is done packing it.

So the real, safe send size in desktop Outlook is closer to 14 to 15 MB. On Outlook.com it's roughly 18 MB. Anything above that is rolling the dice.

And there's a second catch: the recipient's server has its own limit. Corporate Exchange servers often cap incoming mail at 10 to 15 MB. Your email can leave Outlook without complaint and still bounce at the other end, hours later, in a delivery failure notification.

The new Outlook quietly uploads your files to OneDrive

If you're on the new Outlook for Windows (the one Microsoft has been rolling out to replace classic Outlook), attaching a large file triggers a different behavior entirely.

Drop a 40 MB video into an email and the new Outlook doesn't give you a “file too large” error. It uploads the file to your OneDrive in the background and replaces the attachment with a share link. The recipient sees a blue button in the email; clicking it opens the file in OneDrive.

This works up to 250 GB per file, which is the SharePoint/OneDrive ceiling. That sounds great until you hit the real problems:

• The recipient might need permission. If your share link is set to “people in your organization,” anyone outside gets an access-denied screen. That includes clients, freelancers, and your personal Gmail.
• It eats your OneDrive quota. Business accounts get 1 TB each, but personal Microsoft 365 Family shares 1 TB across six people. Every email attachment lives there forever until you delete it.
• The file isn't a real attachment. Recipients can't open it offline. If you ever delete the OneDrive file or change permissions, every email that linked to it breaks.
• External recipients may need a Microsoft account. Unless you explicitly set the link to “anyone,” the recipient has to sign in.

Files Outlook won't send, no matter how small

Outlook blocks around 70 file extensions it considers dangerous. The list includes most things that can execute code: .exe, .bat, .cmd, .com, .msi, .vbs, .js, .ps1, .jar, .iso, plus several dozen obscure scripting formats.

Workarounds that don't work:

• Renaming the extension. Outlook's Exchange backend inspects the file content, not just the name.
• Zipping the file. Outlook and Exchange both open ZIP files and scan what's inside. A blocked file in a ZIP rejects the whole email.
• Password-protecting the ZIP. This used to work. Microsoft 365's Safe Attachments now detonates the archive in a sandbox, which means if it can't open it, your message may still be quarantined as suspicious.

The only reliable workaround is to send a download link from somewhere Outlook doesn't inspect.

How Outlook compares to other email providers

Outlook isn't unusually strict. Every major email provider sits in the 20 to 25 MB range. What makes Outlook different is the split between consumer and business: a colleague on a well-funded Microsoft 365 tenant might be able to receive a 100 MB attachment from you while your mother-in-law on free Outlook.com bounces anything over 20.

What's too big for Outlook in real life

“25 MB is generous” is a fine theory until you try to actually use it:

The easiest fix: send a link instead

You don't need a registry edit, a conversation with IT, or a OneDrive subscription. Upload the file once, paste the link into your email, done.

That's what sto.care does. You drop a file (up to 5 GB), and you get a link. Paste it into Outlook, Teams, any email client. The recipient clicks and downloads. No Microsoft account required, no “request access” screens, no OneDrive quota to manage. The file auto-deletes after 7 days.

We put together a full comparison of 22 file sharing services if you want to see all your options side by side.

Frequently asked questions

What is the Outlook attachment size limit?

Free Outlook.com caps at 25 MB per email. Desktop Outlook with a personal account is 20 MB. Microsoft 365 business mailboxes default to 35 MB and can be raised by an admin up to 150 MB. The new Outlook for Windows sidesteps the limit by auto-uploading large files to OneDrive.

Why does my 20 MB file fail to send?

Email packaging adds about a third to the file size. A 20 MB file becomes roughly 27 MB in transit, which overshoots the 25 MB Outlook.com cap. And your recipient's server may be tighter than yours. A lot of company Exchange servers are set to 10 or 15 MB.

Can I increase the limit?

On free Outlook.com, no. On desktop Outlook with Exchange, only your IT admin can change it, and 150 MB is the hard ceiling in Microsoft 365. Registry tweaks on the client only affect the client. The server still rejects oversize messages.

What happens in the new Outlook with a large file?

It uploads the file to your OneDrive and replaces the attachment with a share link. Works up to 250 GB per file but often creates permission problems for recipients outside your organization.

What file types does Outlook block?

About 70 extensions, including .exe, .bat, .cmd, .vbs, .js, .ps1, .msi, .iso, and .jar. Zipping doesn't help. Outlook scans inside archives.

Is the limit per file or per email?

Per email. It covers the message body, inline images, and all attachments combined.

How do I send a large file through Outlook without OneDrive?

Upload to a sharing service like sto.care and paste the link. No Microsoft account needed, no permissions to juggle, no OneDrive storage eaten. See our comparison of 22 services for alternatives.

Does Outlook compress attachments?

It offers to resize images at send time, shrinking large photos to roughly 1024 pixels on the long edge. It doesn't compress other file types.