Hi,
You can develop WCF Rest service and call them from your android application.
it simple your can even migrate your Service already developed by appending simple extra lines of code.
Example for sending mail:
[OperationContract]
[WebInvoke(Method="GET",
ResponseFormat = WebMessageFormat.Json,
RequestFormat = WebMessageFormat.Xml,
BodyStyle = WebMessageBodyStyle.Wrapped,
UriTemplate = "SendMail/{FName}/{LName}/{Phone}/{Email}")]
string SendMail(string FName,string LName,string Phone,string Email);
In WCF Service you can easily connect to sql server with help of ADO.Net connections.
Hope this address your problem.