HOTFIX: .NET 4.0 (System.Net.Mail) Unable to send emails with large attachments (more than 3MB)
We have an official release for the issue and if you are facing this problem then I would recommend you contacting PSS and refer to this blog
I am expected it to be released in a public KB in some time but for the time being you can request for the hotfix by contacting PSS
UPDATE - 7/21/2010
If you prefer not to contact PSS, you can download the hotfix by yourself from the following link.