Increasing Email Attachment Size Limit in Outlook

You may receive an error when attaching files to an Outlook e-mail message:

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 Outlook client, including the latest versions like Outlook 2019, LTSC 2021, and Outlook for Microsoft 365, has a default maximum attachment size of 20 MB. This limitation is within the Outlook client itself, not the mailbox server. If you try to attach a file exceeding 20 MB or multiple files totaling 20 MB, the email won’t be delivered, and you’ll encounter an error.

To increase the maximum size of an attachment in Outlook, you can edit the Windows registry by following these steps:

1. Close the running outlook.exe process;
2. Open the Registry Editor (regedit.exe);
3. To increase the maximum attachment size for the current user, go to the reg key HKEY_CURRENT_USER\Software\Microsoft\Office\xx.x\Outlook\Preferences
   In this registry path, the xx.x value depends on the version of Outlook and Microsoft Office that is installed on your computer:
   16.0 – for Office 2021 LTSC/2019/2016 and Outlook for Microsoft 365
   15.0 – for Office 2013
   14.0 – for Office 2010
   12.0 – for Office 2007
   For example, we have Office/Outlook 2019 installed on our computer, so we need to go to the registry key HKCU \Software\Microsoft\Office\16.0\Outlook\Preferences. Create this registry key manually if it is missing.
4. Create a new 32-bit DWORD registry parameter with the name MaximumAttachmentSize. As its value, enter the maximum size of attachments allowed in Outlook (in KB). For example, set the decimal value to 40960 to allow attachments of files up to 40MB. If you set this value to 0, the maximum attachment size limit in Outlook will be disabled, allowing you to attach files of any size.
   
   Note. You can use PowerShell to create this registry parameter:

   $regPath = "HKCU:\Software\Microsoft\Office\16.0\Outlook\Preferences"
   
   New-ItemProperty -Path $regPath -Name MaximumAttachmentSize -Value 40960 -PropertyType DWord –Force

The MaximumAttachmentSize registry option doesn’t affect the allowed size of attachments in Outlook for user mailboxes hosted on Exchange Server or Exchange Online (Microsoft 365).

By default, the maximum size of an outgoing e-mail message with attachments in Exchange Online is limited to 35MB. This parameter is set in the EOL mailbox plan settings. Get the current maximum send email size in EOL using PowerShell:

Get-MailboxPlan | fl name,maxsendsize

You can increase the size of an email in Outlook for a specific Microsoft 365 user:

1. Sign-in to the Microsoft Admin Center;
2. Navigate to Recipients > Mailboxes > select a user mailbox;
3. Go to Maibox tab > Mail flow setting > Message size restrictions;
4. Here you can set the maximum size of an email in Outlook (no larger than 153MB).

In on-premises Exchange Server, message size limits are configured in Transport service settings.