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Outlook: The attachment size exceeds the allowable limit (fix)

Short answer

Outlook shows this when the files attached to one email add up to more than Outlook or your mail server allows. That is 20 MB for POP/IMAP accounts in classic Outlook for Windows, or your Exchange/Microsoft 365 send limit (often 10 MB on older Exchange servers, 35 MB by default in Microsoft 365). The fastest fix is to upload the file to OneDrive and send a link.

What this message means

Outlook checks the combined size of everything attached to a message before it lets you send. If the total goes over the limit, it blocks the attachment and shows this message. Newer versions word it as "The file you're attaching is bigger than the server allows." There are two separate limits. For POP3 and IMAP accounts (Gmail, Yahoo, your internet provider), classic Outlook for Windows sets its own 20 MB cap. For Exchange and Microsoft 365 work or school accounts, Outlook uses the maximum send size your administrator set on the server. The limit applies to the total of all attachments, not to each file separately.

Common causes

CauseHow to tell
A single file is larger than the limit (videos, large PDFs, design files, uncompressed photos).The error appears as soon as you attach that one file. In File Explorer (right-click the file, then Properties) or Finder (Get Info), the size is above 20 MB, or above your work account's limit.
Several attachments add up to more than the limit, even though each one is small.The first few files attach fine and the error only appears when you add the last one. Add up the sizes shown next to each attachment in the message.
Your work or school Exchange server has a low send limit (older Exchange Server installs default to 10 MB).Your account is a company or school address. Files around 10 to 20 MB fail even though a personal Gmail or Outlook.com account could send them, and coworkers see the same error with the same file.
Classic Outlook's built-in 20 MB cap for POP3/IMAP accounts, which is lower than some providers allow.You use Gmail, Yahoo or an ISP mailbox through classic Outlook for Windows, and the same file attaches fine in the provider's own website (for example, Gmail in a browser takes up to 25 MB).
You are forwarding or replying to a message that already carries large attachments or embedded images.You only added a small file, but the forwarded message already lists attachments, or has large pictures pasted into the body.

How to fix it

1. Send a OneDrive or cloud link instead of the file

  1. Remove the attachment that triggered the error.
  2. Upload the file to OneDrive, SharePoint, Google Drive, Dropbox or a similar service.
  3. In classic Outlook for Windows, you can instead go to Insert (or Message) > Attach File > Browse Web Locations > OneDrive, pick the file and choose Share link.
  4. In new Outlook and Outlook on the web, select Attach > Upload to OneDrive (Outlook also offers to share a link when a file is too big), then choose Share link.
  5. Check that the link's sharing permission lets your recipient open the file, especially if they are outside your organization.
  6. Send the email. Only the link counts toward the size limit.

2. Make the attachments smaller

  1. Remove any attachments the recipient doesn't need, including ones carried over from a forwarded message.
  2. For photos in classic Outlook for Windows, open the message in its own window, go to File > Info and turn on Resize large images when this message is sent.
  3. For PDFs, save or export a reduced-size copy (in Adobe Acrobat: File > Save as Other > Reduced Size PDF).
  4. To zip files on Windows, right-click them and choose Send to > Compressed (zipped) folder (on Windows 11, Compress to ZIP file). On a Mac, right-click and choose Compress.
  5. Zipping helps a lot with documents and spreadsheets, but barely shrinks JPG, MP4 or already-compressed PDF files. Use a link for those.

3. Split the files across several emails

  1. Group the attachments so each group stays comfortably under the limit (for example, under 15 MB for a 20 MB cap).
  2. Send the first email with the first group, and number the subject lines, for example "Photos (1 of 3)".
  3. Repeat for the remaining groups.
  4. If one single file is still too big on its own, use a cloud link (fix 1) instead.

4. Raise Outlook's 20 MB limit for POP/IMAP accounts (registry)

Classic Outlook for Windows 2013, 2016, 2019, 2021, 2024 and Microsoft 365, with POP3/IMAP accounts only. Not available in new Outlook, Outlook on the web or Outlook for Mac.

  1. Find your provider's real limit first (Gmail and Yahoo allow about 25 MB). Raising Outlook's limit above the server's limit only means the email bounces after sending.
  2. Close Outlook, then press Windows+R, type regedit and press Enter. Back up the registry before making changes.
  3. Go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences (use 15.0 for Outlook 2013). If the key doesn't exist, create it.
  4. Right-click in the right pane, choose New > DWORD (32-bit) Value and name it MaximumAttachmentSize.
  5. Double-click the value, select Decimal and enter the limit in KB, for example 25600 for 25 MB. A value of 0 removes Outlook's limit, but that isn't recommended.
  6. Close Registry Editor and start Outlook again.

5. Ask your administrator to raise the Exchange or Microsoft 365 send limit

Work or school accounts on Exchange Server or Exchange Online (Microsoft 365). Requires admin rights.

  1. Send your IT team the file size you need and the exact error text.
  2. In Microsoft 365, an admin can open the Exchange admin center > Recipients > Mailboxes, select the user, and under Mail flow settings, edit Message size restrictions. Any value from 1 MB to 150 MB is allowed (the default is 35 MB to send and 36 MB to receive).
  3. An admin can also do this in PowerShell: Set-Mailbox -Identity user@domain.com -MaxSendSize 50MB -MaxReceiveSize 50MB.
  4. For on-premises Exchange, the admin changes the organization's Maximum send size in the transport settings.
  5. The change can take several hours to reach Outlook because the server caches these settings. Restart Outlook after waiting.

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Still not fixed?

If a cloud link isn't an option and the file is still blocked, find out which limit you're hitting. Try sending the same file from your provider's website (Outlook on the web, Gmail in a browser). If it goes through there, the cap is in classic Outlook, and the registry fix applies. If it fails there too, the server's limit is the problem, and only your email provider or IT administrator can change it. Also keep in mind that the recipient's server has its own limit. If you get a bounce message such as "552 message size exceeds fixed maximum message size" after sending, the recipient's side rejected it, and a download link is the only reliable fix.

Questions people ask

What is the maximum attachment size in Outlook?

Classic Outlook for Windows allows 20 MB in total for POP/IMAP accounts. Work and school accounts use the Exchange limit: 35 MB by default in Microsoft 365 (an admin can raise it to 150 MB) and often 10 MB on older on-premises Exchange servers. Files shared as OneDrive links don't count toward these limits.

Why does my 18 MB file still bounce even though it is under the limit?

Email encoding makes attachments roughly 33% larger on the way out, so an 18 MB file becomes about 24 MB in transit. It can then go over your server's or the recipient's limit, even though Outlook let you attach it.

Can I increase the attachment size limit in Outlook?

For POP/IMAP accounts in classic Outlook for Windows, yes, with the MaximumAttachmentSize registry value. For Exchange or Microsoft 365 accounts, only an administrator can raise the limit on the server. Outlook for Mac, new Outlook and Outlook on the web have no user setting for it.

Does zipping a file get it under the limit?

Only for files that compress well, like Word, Excel, text and CAD files. Photos, videos and most PDFs are already compressed and shrink very little. Send those as a cloud link instead.

Is the limit per file or per email?

Per email. Outlook adds up the sizes of all the attachments in the message, including ones carried over when you forward, and compares the total with the limit.

Sources

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