What is Outlook's attachment size limit?

There is no single answer, which is the whole problem. Outlook.com sits at around 25 MB. Microsoft 365 defaults to about 35 MB per message and an administrator can raise it to roughly 150 MB. An on-premise Exchange server is whatever your IT department configured, and 10 MB defaults are still common in the wild.

How do I increase the attachment size limit in Outlook?

On a personal account, you cannot. On Microsoft 365, an administrator can raise the organisation's message size limit through the Exchange admin centre or PowerShell, up to around 150 MB. It is worth remembering that raising your own limit does nothing about the recipient's, which is usually the one that rejected the message.

Why can Outlook send a file that then bounces?

Because two limits apply and they belong to different organisations. Yours decides whether the message leaves; the recipient's decides whether it is accepted. A 40 MB attachment can pass a generously configured Microsoft 365 tenant and be refused by a mail server on the other side that caps messages at 10 MB.

How do I send large files in Outlook?

Attach the file and Outlook offers to upload it to OneDrive and send a link instead. That is the built-in route. Check the sharing permission it applies, and remember the link stays live in your OneDrive until you remove it, which matters for client work that has since finished.

Does OneDrive count against the attachment limit?

No. A OneDrive link is a few hundred bytes of text in the message, so the size of the file behind it is irrelevant to any mail server. That is exactly why sending links rather than attachments sidesteps this entire category of problem.

Outlook's Attachment Size Limit: 20, 25 or 150 MB?

Outlook has no single limit — it depends on your account and your administrator. Here is how to find your real number, and what to do when it is too low.

Gmail has one number. Outlook has four, and which one applies to you depends on things you may not be able to see.

Your account	Typical limit
Outlook.com (personal)	Around 25 MB
Microsoft 365 (business, default)	About 35 MB per message
Microsoft 365 (administrator raised)	Up to around 150 MB
Exchange, on-premise	Whatever IT set — 10 MB is still common

If you want the real number for your own account, the reliable test is not a support page. It is to attach a file of a known size and see what happens, because your administrator’s configuration beats any documentation.

The number that actually stops you belongs to someone else

Here is the part that catches out even people who have looked their own limit up.

Your limit decides whether the message leaves. The recipient’s decides whether it arrives. The smaller of the two wins, and you have no visibility of theirs.

So the story goes: your administrator raised the tenant limit to 150 MB, you cheerfully attach a 60 MB video, Outlook sends it without complaint, and the client’s mail server — a different company, different rules, a default from years ago — refuses it at the door. Your sent folder shows success. Sometimes a bounce arrives. Often it just sits in a quarantine queue that nobody reviews.

If you send to businesses, assume the tightest limit in the chain, not yours.

And the size Outlook measures is not the size on disk

Attachments are encoded into the message body in base64, which inflates them by roughly 37%. The message text, the headers and your signature image all count as well.

Against a 25 MB ceiling that leaves about 18 MB of real file. Against a 35 MB one, about 25 MB. “It is 30 MB and my limit is 35 MB” is not the comfortable margin it looks like.

There is also a separate Outlook desktop behaviour worth knowing: on some configurations the client refuses very large attachments with a message about the file being too large before the server is ever consulted. It is the same problem wearing a different hat.

Raising the limit, if you can

Personal Outlook.com: you cannot. The limit is the limit.

Microsoft 365 with admin rights: the message size limit can be raised in the Exchange admin centre, or set with PowerShell, up to roughly 150 MB. Two caveats before you do it. It does nothing about the recipient’s limit, which is usually the one rejecting your mail. And every message that size has to be stored, scanned and backed up by both organisations, which is precisely why the low default exists.

On-premise Exchange: ask whoever runs it. Expect a reasonable no.

Honestly, the whole exercise usually confirms that raising a limit is the wrong fix for a file that should not have been an attachment.

What to do instead

Send the OneDrive link. Attach an oversized file in Outlook and it offers to upload to OneDrive and insert a link. This is the built-in answer and it works. Watch the sharing permission it applies, and remember the link stays live until you revoke it by hand — a project that finished a year ago still has an open door if nobody closed it.

Compress properly. Most oversized PDFs contain print-resolution imagery inside a document nobody will print; a screen-resolution export routinely takes 40 MB under 5 MB. Video and photos will not benefit from zipping. And a zip is the file type most likely to be stripped by a corporate filter, which turns “too big” into “arrived empty”.

Use a transfer link for genuinely large files. Options compared here.

Stop attaching things people only read. Proposals, reports, brochures and decks do not need to be files. A link opens on any phone, cannot bounce, can be corrected after sending, and tells you when it was read.

If you send client work every week

Attachment limits are an annoyance once and a workflow problem repeatedly. If large client deliveries are a regular part of the job, the useful questions stop being about megabytes: can you set an expiry, can you revoke a mis-send, do you know when it was collected, and does the download page look like it came from you?

We built Transfers for that, and it is coming soon on the Living Page Agency plans.

I’m Richard, and I build Living Page — send documents and large files as links that open anywhere and tell you when they land. Try it free.