Outlook's attachment size limit, version by version

What SendTheStuff will do when Outlook's cap is too small

SendTheStuff will turn a file of up to 5 GB into one link for any Outlook message. The link is a single line of text, so it fits under every cap in the table below. That includes an administrator's lowest possible setting, 1 MB.

It will help most where Outlook's ceiling sits lowest:

• A POP or IMAP address in classic Outlook, held to 20 MB unless you edit the registry. With a link, there's nothing to edit.
• Outlook on the web at work, which caps messages at 25% less than the configured setting.
• Mail to people outside Microsoft's datacenters, where even a maxed-out work limit stops at 112 MB.
• A recipient whose inbox takes less than yours. A link arrives whatever their mail server accepts.

Nothing will be zipped or resized, and nothing will count against your OneDrive storage. A SendTheStuff link will last 14 days; after that, the file is gone for good.

Outlook's attachment limit in every version

The number depends on two things: the mailbox you send from, and the Outlook app you send it in.

A few rows need a footnote:

• New Outlook for Windows. Microsoft describes the new Outlook as a native app whose experience comes from Outlook on the web. It lists no attachment figure of its own.
• Outlook for Mac. Microsoft documents Mac limits for Microsoft 365 mailboxes only. The new Outlook for Mac follows your mobile device policy and honors "the lowest value among MaxAttachmentSize, MaxSendSize, and MaxRequestSize".
• Phones with Outlook.com. The app's help lists 33 MB, but Outlook.com's own page caps an email at 25 MB. Plan on the lower number.

How to tell which limit is yours

Start with the address you send from, not the app:

• A personal Microsoft address, such as one ending in @outlook.com, is Outlook.com. Its cap is 25 MB per email.
• An address your employer or school gave you almost always sits on Microsoft 365 or an Exchange server. Only IT knows the exact figure; the defaults are in the table.
• Another provider's address added to classic Outlook, like Gmail or your internet provider's mail, runs on POP or IMAP. Outlook's 20 MB check applies first, then the provider's own cap.

Not sure which kind a mailbox is? Whoever set it up, you or your IT team, chose POP, IMAP or Exchange when adding it.

The exact error in each Outlook

Classic Outlook for Windows shows the same box for both kinds of limit, its own POP and IMAP cap and an Exchange server's limit:

The file you're attaching is bigger than the server allows. Try putting the file in a shared location and sending a link instead.

Outlook on the web connected to a company's own Exchange Server (2013 or 2016) says something different:

The following files weren't attached because adding them would cause the message to exceed the maximum size limit of 10 MB: filename.

Microsoft 365 behaves differently again. A message over the limit isn't delivered, and you get a non-delivery report (NDR) back in your inbox.

Microsoft doesn't publish the exact wording Outlook.com or the new Outlook shows. Whatever the message says, compare the file with your row in the table above.

Why a file under the limit is still refused

The cap you read in the table isn't always the size you can actually attach:

• Encoding adds weight. Email converts attachments to text, which makes them about 33 percent larger in transit. A file just under the cap can end up over it.
• Every attachment counts. Classic Outlook checks the sum total of all your files, not the biggest one. Outlook.com counts the message text too.
• Outlook on the web at work holds some back. With a Microsoft 365 mailbox, it caps messages at 25% less than the configured setting, to leave room for that encoding.
• The recipient has a limit too. A Microsoft 365 inbox takes 36 MB by default, and other mail servers set their own.
• Leaving Microsoft costs room. If IT has raised your limit above 112 MB, mail to outside servers still stops there.
• Some apps are stricter. Microsoft notes that "Some email clients may have lower message size limits".
• Too many files. Exchange Online allows 250 attachments per message, and admins can add transport rules capping any single file.

Can you raise Outlook's attachment limit?

Sometimes, depending on who controls the limit.

Outlook.com. Microsoft documents no way to raise the 25 MB cap. For a bigger file, send a link instead.

Microsoft 365 at work or school. Only an administrator can change it. In the Exchange admin center, they open Recipients > Mailboxes, select your mailbox and choose Manage message size restriction. Any value from 1 MB to 150 MB works.

In PowerShell, the same settings are MaxSendSize and MaxReceiveSize. Microsoft says limits can be "configured up, down, or on a per-user basis", so IT can raise yours alone. Even at the top setting, outside recipients still stop at 112 MB, and Outlook on the web at 112 MB.

On-premises Exchange Server. The 10 MB default belongs to the server, so the Exchange administrator changes it there.

Classic Outlook with a POP or IMAP address. This is the one limit you can change yourself. Microsoft's registry route:

1. Back up the registry first, as Microsoft advises.
2. In Registry Editor, open HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Preferences. The x.0 is 16.0 for Microsoft 365 and Office 2016 or later, or 15.0 for Office 2013.
3. Add a DWORD value named MaximumAttachmentSize. Its data is in kilobytes, entered as a decimal; Microsoft's example is 30720.
4. A value of 0 removes Outlook's own check entirely. Close Registry Editor and restart Outlook.

The edit only moves Outlook's check. Your provider's cap, and your recipient's, stay exactly where they were. For an Exchange or Microsoft 365 mailbox, Microsoft says "the 20-MB attachment limit for Internet email accounts is not used", so the change does nothing there.

Often the quicker fix is to raise nothing at all and send the file from Outlook as a link instead.

Why the usual workarounds fall short

Each of Outlook's own routes stops somewhere:

OneDrive is the strongest of these, but it's storage, not delivery. Every big file you share stays until you tidy it up. And a registry edit or an IT ticket only moves one limit in a chain of several.

Until SendTheStuff launches

Share the file from OneDrive instead of attaching it. Each Outlook words the button differently:

• Outlook.com: choose Attach, then Upload and share, then Share link.
• Outlook on the web, with a file already in OneDrive: pick it under Browse cloud locations, then Share as a OneDrive link.
• New Outlook for Windows: attach the file, then choose Upload to OneDrive.
• Classic Outlook: attach the file, then choose Upload > OneDrive.

Outlook.com won't link a OneDrive file over 2 GB. Anything larger needs another cloud service's share link, pasted into the message.

When it still won't send

• The error appears for a file below your limit. Add up every attachment, and allow for the about 33 percent that encoding adds. Then send a link.
• IT raised your limit, but Outlook on the web still refuses. With Microsoft 365, it caps messages at 25% less than the configured setting. In classic Outlook, the mailbox's own limit applies; IT can set it as high as 150 MB. Or send a link.
• It sends inside the company but bounces outside. If IT raised your limit above 112 MB, outside mail still stops at that figure. The recipient's server may take less, too.
• The registry edit changed nothing. Either your mailbox is Exchange or Microsoft 365, where Outlook uses the server's limit instead. Or your POP or IMAP provider's own cap is lower than your new setting.
• You shrank a photo and it's still too big. Microsoft says "Resizing an image doesn't reduce its file size." Compress the file instead, or send a link.
• The phone app refuses an iCloud attachment. Outlook for iOS and Android caps iCloud mail at 20 MB.
• A folder of small files won't attach. Exchange Online stops at 250 attachments per message. Zip them into one file first.