Blog · 24 August 2026

“The attachment size exceeds the allowable limit”: fixing the Outlook error

20 MB on a POP/IMAP account, 10 MB on Exchange: three limits stacked on top of each other, and a registry key that is not enough.

The exact message, and where it comes from

Outlook shows one or other of these wordings:

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

The refusal comes from the software, before anything is sent. Microsoft designed it that way so your machine does not spend hours uploading an attachment the provider will reject anyway.

The three limits stacked on top of each other

This is the explanation missing everywhere, and the reason so many people edit the registry to no effect.

The lowest one always decides. Raising Outlook's simply moves the refusal: instead of an immediate error message, you will get a non-delivery report an hour later.

Raising the software limit (POP3 or IMAP account)

The procedure is documented by Microsoft. It touches the Windows registry: back it up first.

1. Close Outlook.
2. Open the Registry Editor (regedit).
3. Go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences — 16.0 for Office 2016, 2019, LTSC 2021 and Microsoft 365; 15.0 for Office 2013. Create the path if it does not exist.
4. Add a DWORD value named MaximumAttachmentSize.
5. Give it the size you want in kilobytes: 30720 in decimal for 30 MB. A value of 0 removes the software limit entirely.
6. Close the editor and restart Outlook.

On an Exchange account this key is ignored: the limit comes from the server's maximum send size setting, which only the administrator can change.

One last factor works against you, and this one is invisible: the attachment is base64-encoded before sending, which makes it about 33 % heavier. A 20 MB file on disk weighs nearly 27 inside the message.

OneDrive: Microsoft's answer, and its limits

Outlook offers to upload the file to OneDrive and send a link. That is the right reflex for a file of a few dozen megabytes. Beyond that, two obstacles: the file eats into your OneDrive space, and the recipient sometimes hits a request to sign in to a Microsoft account — especially if your company restricts external sharing, which is the default setting in many organisations.

Sending a file of several gigabytes

For rushes, a set of photos or a project archive, the attachment is no longer the question: you need a link that works for anyone.

toobigtoshare accepts up to 2 TB. You drop the file, you get a link, you paste it into your Outlook message. The recipient clicks and downloads: no Microsoft account, no permission to request, no sharing setting to negotiate with your IT department. $5.49 per send, file erased after 3 days.

Send the file with toobigtoshare — up to 2 TB, no account, a link to paste into your email.

Frequently asked questions

What is the attachment limit in Outlook?

20 MB (20,480 KB) by default for an internet mail account over POP3, IMAP or HTTP, and 10 MB (10,240 KB) by default for an Exchange account. On Microsoft 365 the default message limit is 35 MB, which the administrator can raise to 150 MB.

How do I increase the attachment limit in Outlook?

For an internet account, by adding a DWORD value named MaximumAttachmentSize under the key HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences, expressed in KB. A value of 0 removes the software limit — but not the server's, which will refuse anyway.

Why does the error persist after editing the registry?

Because Outlook's limit is only the first of three. Your sending server applies its own, and so does the recipient's server. The lowest of the three always decides.

How do I send a file of several GB with Outlook?

By sending a link rather than an attachment. OneDrive handles it within the limits of your subscription; a transfer service such as toobigtoshare accepts up to 2 TB without the recipient having to create a Microsoft account.