Some Outlook users had a problem sending large attachments:

When I attach several image files to an email in Outlook I get 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."

Users with older versions of Outlook may receive this message instead:

"The attachment size exceeds the allowable limit".

In order to reduce NDRs and failed sends, Outlook checks the message size before sending and alerts the user that the message is larger than the mail server allows. Because Outlook can't query a SMTP for the maximum attachment size, it uses a default attachment limit of 20 MB (20480kb), which is about right for most ISPs.

This limit eliminates the problem with stuck messages: If you try to send a message using SMTP, you get an NDR back into your Inbox telling you the message is too large. Outlook wastes a lot of time uploading the huge file that will never send and this prevents the sending of any other mail.

Exchange server accounts should use the message size set for the user account. Exchange 2010 users will see a mailtip above the To field instead of the warning dialog.

If the MaximumAttachmentSize is set higher on the client than the Exchange account is allowed to send, the message will not send. The Send and Receive Progress Dialog will show this error:

Task 'alias@domain.com - Sending' reported error (0x80040610) : 'The message being sent exceeds the message size established for this user.'

If you can send larger attachments you can edit the registry to allow larger limits. While you can set it to 0 for unlimited message size, its better to set the limit to the size your ISP allows.

Open the registry editor and browse to:

Outlook 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

Then add a new DWORD value: MaximumAttachmentSize

Unlimited message size uses a value of 0. For a specific message size (recommended) enter the decimal value in Kb. You may want to round the value down to allow for overhead, so 50MB = 50000.

If your mail server limits you to smaller attachment sizes, you can set the key to prevent an NDR when you attempt to send messages that are too large.

Instant Fix

You can can change the dword value in code below if needed (its set for 30 MB) then copy and paste into notepad and save as "MaximumAttachmentSize.reg". You'll double click on the saved file to install it in your registry.

Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences]
"MaximumAttachmentSize"=dword:00007530

Download a ready-to-run registry file to set the maximum attachment size to 30 or 50 MB:
Outlook 2016 50MBOutlook 2013 50MB
Outlook 2010 50MB Outlook 2007 50MB

Outlook 2016 30MBOutlook 2013 30MB
Outlook 2010 30MB Outlook 2007 30MB

Email Compression Tools

If you use tools such as WinZip's Companion which compress large attachments automatically after you add them to the email message, you'll want to set the maximum allowed size at least twice as large as allowed by your email service, or set it to 0 to disable completely. Outlook checks the attachment size as soon as you add the attachments, not before you send the message.