[app-wiring-refactor]: Apply review comments:

- Add reference to original library in `FluentBuilder`.
- Change `I<Name>` notation to `<Name>Impl` notation.
- Move error config to test resources
- Add comment with config instruction
- Add config to the documentation

Mage.Server/src/main/java/mage/server/MailgunClientImpl.java

@@ -0,0 +1,45 @@
+package mage.server;
+
+import com.sun.jersey.api.client.Client;
+import com.sun.jersey.api.client.ClientResponse;
+import com.sun.jersey.api.client.WebResource;
+import com.sun.jersey.api.client.filter.HTTPBasicAuthFilter;
+import com.sun.jersey.core.util.MultivaluedMapImpl;
+import mage.server.managers.ConfigSettings;
+import mage.server.managers.MailClient;
+import org.apache.log4j.Logger;
+
+import javax.ws.rs.core.MediaType;
+
+public class MailgunClientImpl implements MailClient {
+
+    private static final Logger logger = Logger.getLogger(Main.class);
+
+    private final ConfigSettings config;
+
+    public MailgunClientImpl(ConfigSettings config) {
+        this.config = config;
+    }
+
+    public boolean sendMessage(String email, String subject, String text) {
+        if (email.isEmpty()) {
+            logger.info("Email is not sent because the address is empty");
+            return false;
+        }
+        Client client = Client.create();
+        client.addFilter(new HTTPBasicAuthFilter("api", config.getMailgunApiKey()));
+        String domain = config.getMailgunDomain();
+        WebResource webResource = client.resource("https://api.mailgun.net/v3/" + domain + "/messages");
+        MultivaluedMapImpl formData = new MultivaluedMapImpl();
+        formData.add("from", "XMage <postmaster@" + domain + '>');
+        formData.add("to", email);
+        formData.add("subject", subject);
+        formData.add("text", text);
+        ClientResponse response = webResource.type(MediaType.APPLICATION_FORM_URLENCODED).post(ClientResponse.class, formData);
+        boolean succeeded = response.getStatus() == 200;
+        if (!succeeded) {
+            logger.error("Error sending message to " + email + ". Status code: " + response.getStatus());
+        }
+        return succeeded;
+    }
+}