InetSendMail does not support TLS (or SSL). If you do not mind using .NET 2.0, then the System.Net.Mail.SmtpClient class in the System assembly lets you do this - this class is new to .NET 2.0. You can call .NET assemblies from CVI by creating a wrapper for the assembly using Tools >> Create .NET Controller - it will take a while to generate and compile the wrappers for larger assemblies. If you cannot or do not want to use .NET then you may be able to find open-source code or C libraries that do TLS.