How to Increase the Outlook Attachment Size Limit

Match Outlook's size limit to your mail server's size limit

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

Instructions in this article apply to Outlook 2019, Outlook 2016, Outlook 2013, Outlook 2010, and Outlook for Office 365.

How to Increase the Outlook Attachment Size Limit

When sending an email attachment in Outlook, you may receive an error message warning you that the attachment size exceeds the allowable limit. When your mail server allows messages up to 25 MB and your attachment is slightly over the default 20 MB limit, change Outlook's default to match the mail server's default size.

Before making changes to the Windows Registry, back up the Registry so that if you make changes, you can restore your system to its original state.

  1. Press Windows+R.

  2. In the Run dialog box, type regedit.

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

  4. Navigate the registry tree and go to the entry corresponding to your Outlook version:

    • Outlook 2019 and 2016: HKEY_CURRENT_USER\­Software\­Microsoft\­Office\­16.0\­Outlook\­\Preferences
    • Outlook 2013: HKEY_CURRENT_USER\­Software\­Microsoft\­Office\­15.0\­Outlook\­\Preferences
    • Outlook 2010: HKEY_CURRENT_USER\­Software\­Microsoft\­Office\­14.0\­Outlook\­\Preferences
    Preferences in Office/16.0 Registry
  5. Double-click the MaximumAttachmentSize value.

    If you don't see MaximumAttachmentSize, add a registry key and value. Go to Edit, select New > DWORD Value, enter MaximumAttachmentSize, and press Enter.

    MaximumAttachmentSize in regedit
  6. In the Value data text box, enter the desired attachment size limit in KB. For example, to set a size limit of 25 MB, first tick the box next to Decimal and then enter 25600 (because 25600 decimal = 25.6 MB).

    25600 value data for Outlook regedit preferences
    • The default value (when MaximumAttachmentSize is not present) is 20 MB or 20480.
    • For no attachment file size limit, enter 0. Most mail servers have a size limit, so 0 is not recommended; you may get large messages back as undeliverable.
    • The limit corresponds to your mail server's limit. Reduce the Outlook limit by 500 KB to allow wiggle room.

    You may be confused that 25600 KB equals 25 MB. That's because regedit uses a different measurement system than you may be familiar with. Because of these differences, regedit uses 1024 KB to equal 1 MB. So, the equation to determine the decimal number depends on the MB of storage you want to use. In this case 25 MB is: 25 x 1024 KB = 25600 KB.

  7. Select OK.

  8. Close the Registry Editor.