@fredyfx este s el codigo que me da mailgun
public static RestResponse SendSimpleMessage() {
RestClient client = new RestClient();
client.BaseUrl = "https://api.mailgun.net/v3";
client.Authenticator =
new HttpBasicAuthenticator("api",
"key-3628d3d724bff943a070b7f6fd0ec80b");
RestRequest request = new RestRequest();
request.AddParameter("domain",
"sandbox59470b3d16f149169488d2923e450749.mailgun.org", ParameterType.UrlSegment);
request.Resource = "{domain}/messages";
request.AddParameter("from", "Mailgun Sandbox <postmaster@sandbox59470b3d16f149169488d2923e450749.mailgun.org>");