However, this error is a bit misleading. Even if you configure your mail server to support large email attachments, Outlook will continue to throw this same error because the 20 MB limit is actually there in the Outlook client, not your mail server. Whether you're attaching a single file or multiple files in an email message, the maximum attachment size in Outlook is, by default, 20 MB.
Increase Outlook attachment size limit with the regedit GUI
The Windows registry plays a critical role in the configuration, management, and tweaking of the Windows OS, as well as the software installed on it. To increase the attachment size in Outlook, launch regedit.exe, and navigate to either of the following registry subkeys:
Where xx.x is a variable value determined by the version of Microsoft Outlook (or Office Suite) installed in your system (e.g., for Office 2021/2019/2016, the variable becomes 16.0). The following table shows the registry path for different versions of Office/Outlook:
|Office / Outlook Version||Registry Path|
|2016, 2019, 2021 or Office 365||HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PreferencesOR|
You don’t need to create the registry value under both subkeys. Creating it under either one of these works just fine.
After navigating to the registry subkey, create a new value (or modify if it already exists) with the following information:
- Value Name: MaximumAttachmentSize
- Value Type: DWORD (32 bit)
- Value Data: 30720 (any integer value)
You can type any number in Value Data to specify the maximum attachment size in KB (30720 in our example, denotes 30 MB). Setting it to 0 disables Outlook's built-in attachment size limit, allowing you to attach file(s) of any size. However, doing so seems impractical, since all mail servers are configured with a maximum supported message size. The following screenshot shows how to create the MaximumAttachmentSize DWORD value using the registry editor GUI tool:
Now close the registry editor, and you're done. If you love doing things with PowerShell, see the next section.
Increase attachment size limit using PowerShell
Since the HKEY_CURRENT_USER and HKEY_LOCAL_MACHINE registry hives are mapped under the HKCU and HKLM PS drives, respectively, we can simply use the New-ItemProperty cmdlet to create the required registry value. To create the MaximumAttachmentSize registry value with PowerShell, fire up a PowerShell console, and type the following commands:
$regPath = "HKCU:\Software\Microsoft\Office\16.0\Outlook\Preferences" New-ItemProperty -Path $regPath -Name "MaximumAttachmentSize" -Value 30720 -PropertyType DWord -Force
Make sure you adjust the $regPath variable as per your Office version and the -Value parameter to match your preferred attachment size limit.
Increase Outlook attachment size limit with Group Policy
If the 20 MB limit of Outlook doesn't suit your organization's needs, you can create the aforementioned registry value using a GPO, as shown in the following screenshot:
Verifying the Outlook attachment size limit
Now reopen Outlook and try to attach the files again to make sure the new attachment size limit is working as expected. The following screenshot shows that I could attach files larger than 20 MB after creating the said registry value:
Keep in mind that this guide will help you increase the attachment size limit in the Outlook client only. The ability of a mail server to send such big emails totally depends upon its configuration, which is usually controlled by the mail server admin. Even if you set your mail server to support large attachments, the final email delivery to the intended recipient is decided by the recipient's mail server in the end. If the destination mail server doesn't support it, you will get a non-delivery report (NDR) letting you know why your message wasn't delivered.
Read Change maximum Microsoft 365 attachment size with PowerShell if your Outlook clients send emails via Microsoft's cloud.