After originally posting this issue on the MS Community Forums, it was suggested by MS Support Engineer Ratandeep that I post this issue in the TechNet forum for additional assistance...

Issue Description:

I have a client using Windows Mail on a Surface Pro tablet running Windows 8.1. From the Windows Mail client, they are unable to send messages with attachments greater than 5 MB in size. If I have them send via Outlook, there's no issue. Based on the Transport Settings configured on our Exchange server, we should be able to send a 50 MB attachment.

Does anyone know of a registry key that can be added (similar to the one for Outlook) that will configure Windows Mail to allow for larger attachments?

Additional information:

You asked: Is Surface Pro connected to a domain network?

-- Yes, it is connected to a domain network. Sometimes on the local wireless network, and other times through a VPN connection. Please note: On our Domain Exchange server, the Transport configuration allows the sending and receiving of attachments of 100 MB or smaller.

You stated: This issue may occur due to corrupt mail app files or settings.

-- If the currently installed app was "corrupt," wouldn't installing the latest version (which I did) fix that issue? As far as settings, I haven't seen any setting in the client or the registry regarding attachment size.

You stated: The maximum size of the file which can be attached in a message in mail app is 25 MB.

-- That's what I had read during my research of the issue, but this user is clearly experiencing an issue with attachments of 5 MB or larger.

• Edited by R.Doyle.5Below 17 hours 46 minutes ago

Hi,

The Mail Client you meant here is refer to Windows built-in Mail APP, isn't it? 

When this problem occurs, is there any error message? I made a test in our testing environment, when single file is larger than 25M, it would prompt an error message: "The attachments might be too large to send this message". So, what's your error message?

On the other hand, to test if it is the mail app problem, you can add another Live Mail Account, then try to send an email with attachment for test. Will them got same results? Please have a try.