SMTP is the basic mail protoc0l. Any mail server (presumably) supports this. According to the Xmail Home Page
it is "an Internet and intranet mail server featuring an ESMTP server, ... TLS support for SMTP and POP3". From this I assume that it meets the criteria for SMTP.
All you should need to do is change your .NET code to point to a running instance of an XMail mail server to send mail using it.