Therefore, users who are authenticated with SQL Server Authentication cannot attach files using @file_attachments. Windows does not allow SQL Server to provide credentials from a remote computer to another remote computer
I don’t think it is related because @file_attachment is local variable in SQL Server not related to system.net used for Microsoft SMTP mail. Please check the link below for the system stored procedure that defines attachment file size and related issues, the reason you can use Default for the connection properties but you still need to talk to system admin and Exchange admin.
http://msdn.microsoft.com/en-us/library/ms191442.aspx
Kind regards,
Gift Peddie