One of the customer reported that whenever he tries to send e-mail by using System.Net.Mail API they get the following error “System.Net.Mail.SmtpException error message: Command not implemented. The server response was: Command not Supported”
I had a look at the application. They built a Microsoft Visual Studio 2005 based application and tried to send e-mail by using the System.Net.Mail.SmtpClient class. They use the DeliveryMethod.Network method from the Microsoft .NET Framework 2.0. The application receive the following System.Net.Mail.SmtpException error message: Command not implemented. The server response was: Command not Supported.
During the investigation, we found that this problem occurs because the remote SMTP server does not support the SMTP extended Hello (EHLO) command. The exception occurs when the remote SMTP server returns the 502 reply code and the System.Net.Mail.SmtpClient class expects the 500 reply code.
To overcome this issue, Microsoft released a hot fix downloaded from http://code.msdn.microsoft.com/KB913616/Release/ProjectReleases.aspx?ReleaseId=789. Also you can refer the detailed Knowledge base article regarding this issue and hot fix.