How to Increase the Outlook Attachment Size Limit

Match Outlook's Size Limit to Your Mail Server's Size Limit

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By default, Outlook does not send email messages with attachments that exceed 20MB, but many mail servers allow 25MB or larger attachments. You can instruct Outlook to send messages larger than the 20MB default as long as your mail server allows it. You can also avoid getting back undeliverable messages if Outlook's default is larger than what you can actually send through your mail server.

Do You Get This Error Message in Outlook?

The attachment size exceeds the allowable limit.


Outlook's refusal to send would be okay if you're attempting to share a 200MB video, but when you know your mail server will let you send messages up to 25MB and your attachment is only slightly over the default 20MB limit, you can change Outlook's default to match the mail server's default size.

Increase the Outlook Attachment Size Limit

To change the size Outlook allows as a maximum for attachments to send:

  1. Press the keyboard shortcut Windows-R.

  2. Type "regedit" in the Run dialog.

    regedit command in Run menu in Windows 10
  3. Click OK.

  4. Travel down the registry tree to the entry corresponding to your Outlook version:

    • Outlook 2010: HKEY_CURRENT_USER\­Software\­Microsoft\­Office\­14.0\­Outlook\­\Preferences.
    • Outlook 2013: HKEY_CURRENT_USER\­Software\­Microsoft\­Office\­15.0\­Outlook\­\Preferences.
    • Outlook 2016: HKEY_CURRENT_USER\­Software\­Microsoft\­Office\­16.0\­Outlook\­\Preferences.
    Preferences in Office/16.0 Registry
  5. Double-click the MaximumAttachmentSize value.

    • If you cannot see MaximumAttachmentSize:
    • Select Edit > New > DWORD Value from the menu.
    • Enter "MaximumAttachmentSize" (not including the quotation marks).
    MaximumAttachmentSize in regedit
    • Press Enter.
    • Now double-click the MaximumAttachmentSize value you just created.
  6. Enter the desired attachment size limit in KB under Value Data:

    • To set a size limit of 25MB, for example, enter "25600."
    25600 value data for Outlook regedit preferences
    • The default value (with MaximumAttachmentSize not present) is 20MB or 20480.
    • For no attachment file size limit, enter "0." Practically all mail servers have a size limit, though, so "0" is not recommended; you would invariably get large messages back as undeliverable after an often long and fruitless uploading process.
    • Ideally, the limit corresponds to your mail server's limit. Reduce the Outlook limit by some 500KB to allow wiggle room.
  7. Click OK.