What is the Outlook attachment size limit?

It depends on the account. Outlook.com (free consumer accounts) caps attachments at 25 MB. Microsoft 365 and Exchange Online default to about 25 MB but an administrator can raise the send limit to 150 MB. On-premises Exchange servers commonly ship with a 10 MB default. The desktop Outlook client applies its own separate 20 MB ceiling for Internet email accounts, which is why some messages are blocked before they ever reach the server.

How do I increase the attachment size limit in Outlook?

For desktop Outlook on Windows, edit the MaximumAttachmentSize value in the registry under the Outlook preferences key and enter the new size in kilobytes — 51200 for 50 MB. Back up the registry first. For Microsoft 365, an administrator raises the organisation limit in the Exchange Admin Center under mail flow settings. Neither change helps if the recipient's server enforces a lower limit.

Why does Outlook say my attachment is too large when it is under 25 MB?

Email attachments are Base64 encoded for transmission, which inflates them by roughly 33%. A 20 MB file arrives at the server as about 27 MB. That encoding overhead is the single most common reason a file that looks comfortably under the limit is rejected — plan for a real ceiling of about 18 MB against a stated 25 MB limit.

How do I send a file larger than 25 MB through Outlook?

Three options, in order of preference: compress the file so it fits, which is the only one that keeps it a genuine attachment; upload to OneDrive and let Outlook insert a share link instead of the file; or split the file into a multi-part archive. Outlook offers the OneDrive link automatically when you attach something oversized.

Does zipping a file get around the Outlook attachment limit?

Only if the contents are compressible. ZIP does very well on documents, spreadsheets, and text-heavy files. It does almost nothing for JPEGs, MP4 video, PNGs, or PDFs full of photographs, because those formats are already compressed — zipping a 30 MB video typically saves under 2%. For media, you need to actually re-encode it, not archive it.

Outlook Attachment Size Limit: What It Actually Is and How to Get Around It

By Hieu DinhPublished August 21, 2026

The Outlook Attachment Size Limit Is Not One Number

Search for the Outlook attachment size limit and you get a confident answer of 25 MB. That number is right for exactly one kind of account and misleading for most of the others.

There are, in practice, two separate limits standing between you and a sent message:

The client limit — enforced by the Outlook application on your machine.
The server limit — enforced by your mail server, and separately by the recipient's mail server.

The smallest number in that chain wins. This is why raising a limit sometimes changes nothing at all: you raised the one that was not blocking you.

Find Your Actual Limit

Account type	Default limit	Can it be raised?
Outlook.com (free)	25 MB	No
Microsoft 365 / Exchange Online	~25 MB	Yes — admin, up to 150 MB
Exchange Server (on-premises)	10 MB	Yes — admin
Outlook desktop client, IMAP/POP account	20 MB	Yes — registry
Gmail account accessed via Outlook	25 MB	No

Note the trap in row four. The Outlook desktop client imposes its own 20 MB cap on Internet email accounts, independent of what your server allows. If you are on Microsoft 365 with a 150 MB server limit and Outlook still refuses a 30 MB file, this client-side ceiling is usually the reason.

The 33% Tax Nobody Mentions

Email cannot carry raw binary data. Attachments are Base64 encoded, which represents every 3 bytes as 4 characters — an increase of roughly 33% before the message leaves your machine.

The consequences are concrete:

File on disk	Size as transmitted	Passes a 25 MB limit?
15 MB	~20 MB	Yes
18 MB	~24 MB	Just barely
20 MB	~27 MB	No
24 MB	~32 MB	No

So against a stated 25 MB limit, your real working ceiling is about 18 MB. Against the desktop client's 20 MB, it is about 15 MB. Size your files against those numbers, not the advertised ones, and the "attachment size exceeds the allowable limit" message stops appearing.

How to Increase the Outlook Attachment Size Limit

Desktop Outlook on Windows (registry)

This raises the client-side cap only. Back up the registry before editing it.

Close Outlook.
Open Registry Editor (regedit).
Navigate to the Outlook preferences key for your version, under HKEY_CURRENT_USER\Software\Microsoft\Office\<version>\Outlook\Preferences.
Find or create a DWORD named MaximumAttachmentSize.
Enter the new limit in kilobytes: 51200 for 50 MB, 102400 for 100 MB.
Restart Outlook.

Setting the value to 0 removes the client limit entirely. That is rarely a good idea — it just moves the rejection later, to the server, after you have waited through the upload.

Microsoft 365 (admin)

An administrator changes this in the Exchange Admin Center under mail flow settings, adjusting the maximum send and receive size on the relevant connector or mailbox plan. The ceiling is 150 MB.

The limitation of both

Neither change affects the recipient's server. You can configure a 150 MB send limit and still be bounced by a client running a 10 MB on-premises Exchange box. Increasing limits helps within an organisation. Across organisations, the only reliable variable is the size of the file itself.

Compress the File Instead

This is the option that keeps the file a real attachment — in the recipient's inbox, in the thread, in the archive — rather than a link that expires or sits behind a permissions prompt.

What matters is knowing which files respond to which treatment.

Files where zipping works

Documents, spreadsheets, CSV, logs, source code. These are text underneath, and text compresses extremely well — a folder of Word documents routinely drops 60–80% in a ZIP. If your attachment is this kind of file, zipping it is the entire fix.

Files where zipping does nothing

JPEG, PNG, MP4, MOV, and most PDFs are already compressed. Running ZIP over them recovers a percent or two. Zipping a 40 MB video to email it is the most common wasted step in this whole exercise.

For these you have to re-encode:

Attachment	What actually works
Photos / screenshots	Re-encode at a sensible resolution and quality — compress images without losing quality
Video	Re-encode to H.264/HEVC at a lower bitrate — compress video for email
PDF	Downsample the embedded images, not the text — how to reduce PDF size
Word / PowerPoint / Excel	Fix the embedded media inside — compress a Word document, reduce PowerPoint file size, compress an Excel file

A 40 MB phone video re-encoded for email typically lands between 5 and 15 MB with no visible difference on a laptop screen. That is the difference between blocked and sent.

When to Use a OneDrive Link Instead

Attach something oversized and Outlook offers to upload it to OneDrive and insert a link. Take the offer when:

The file genuinely cannot be made small enough — a 2 GB video, a large dataset.
Recipients are inside your organisation, where permissions resolve cleanly.
The file is likely to be revised, and you want everyone on the current version.

Decline it when:

Recipients are external, where share links frequently trigger a sign-in wall.
The message needs to stand on its own in an archive, a legal hold, or a compliance record.
The recipient's security policy strips or blocks links to external storage.

Links are convenient and quietly fragile. A compressed attachment has no expiry date, no permissions layer, and no dependency on the recipient having a Microsoft account.

Splitting Archives: The Last Resort

7-Zip and Keka can split an archive into fixed-size volumes — report.7z.001, report.7z.002, and so on — sent across several messages.

It works, and it is genuinely unpleasant for the recipient, who now needs every part present and a tool that can reassemble them. Modern spam filters are also suspicious of multi-part archives. Use it when compression has failed and cloud storage is unavailable, and not before.

A Working Order of Operations

Identify which limit stopped you — client (20 MB) or server (10/25/150 MB).
Subtract the 33% encoding overhead to get your real target size.
Zip it if the file is text-based.
Re-encode it if the file is media — this is where the actual savings live.
Raise the client or server limit if you control both ends.
Use a OneDrive link for genuinely large files going to internal recipients.
Split the archive only if everything above has failed.

Most "attachment too large" problems are solved at step 4, because most oversized attachments are photos and video that nobody re-encoded.

Make the File Fit

Attachment limits have barely moved in fifteen years while camera resolutions and video bitrates have climbed relentlessly. The gap is not closing, and administrators cannot close it for you across organisational boundaries.

Compresto compresses the files that actually cause this problem — video, images, and PDFs — natively on macOS, with batch processing for a folder of attachments and target-size options so you can compress to fit under a specific limit. Nothing is uploaded to a third party, which matters when the attachment is a contract or a client report.

Download Compresto for Mac →