Example send via google domain-wide delegation in GSuite

TestDomainSend.java

import com.google.api.client.googleapis.auth.oauth2.GoogleCredential;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.HttpTransport;
import com.google.api.client.http.javanet.NetHttpTransport;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.jackson2.JacksonFactory;
import com.google.api.client.util.Base64;
import com.google.api.services.gmail.Gmail;
import com.google.api.services.gmail.GmailScopes;
import com.google.api.services.gmail.model.Message;
import com.google.common.collect.Lists;

import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.InputStream;
import java.util.Properties;
import java.util.Random;

import javax.mail.Session;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;

/*
 * Example send email via GSuite service account via domain-wide delegation.
 */
public class TestDomainSend {

  static GoogleCredential createCredential(HttpTransport transport, JsonFactory jsonFactory) throws Exception {
    final String credentialJsonPath = System.getProperty("user.home") + "/.config/gcloud/service-cred-from-console.json";
    try (InputStream credentialStream = new FileInputStream(credentialJsonPath)) {
      final GoogleCredential firstPassCred = GoogleCredential.fromStream(credentialStream, transport, jsonFactory);

      return new GoogleCredential.Builder()
          .setTransport(transport)
          .setJsonFactory(jsonFactory)
          .setServiceAccountId(firstPassCred.getServiceAccountId())
          .setServiceAccountProjectId(firstPassCred.getServiceAccountProjectId())
          .setServiceAccountPrivateKey(firstPassCred.getServiceAccountPrivateKey())
          .setServiceAccountPrivateKeyId(firstPassCred.getServiceAccountPrivateKeyId())
          // service accounts must act on behalf of an actual user and have scope
          .setServiceAccountScopes(Lists.newArrayList(GmailScopes.GMAIL_COMPOSE, GmailScopes.GMAIL_SETTINGS_SHARING))
          .setServiceAccountUser("[email protected]")
          .build();
    }
  }

  public static void main(String[] args) throws Exception {
    Gmail service = createService();
    MimeMessage mimeMessage = new MimeMessage(Session.getDefaultInstance(new Properties()));
    // any address that `[email protected]` has send rights for
    String fromAddress = "[email protected]";
    mimeMessage.setFrom(new InternetAddress(fromAddress));

    String fooMsg = String.format("Hello world! %d", new Random().nextInt(200));
    mimeMessage.setRecipient(javax.mail.Message.RecipientType.TO, new InternetAddress("[email protected]"));
    mimeMessage.setText(fooMsg);
    mimeMessage.setSubject(fooMsg);

    Gmail.Users.Messages messages = service.users().messages();

    try (ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
      mimeMessage.writeTo(baos);
      String encodedEmail = Base64.encodeBase64URLSafeString(baos.toByteArray());
      Message message = new Message();
      message.setRaw(encodedEmail);
      Message sentMessage = messages.send("me", message).execute();
      System.out.println(sentMessage.toPrettyString());
    }
  }


  static Gmail createService() throws Exception {
    final NetHttpTransport transport = GoogleNetHttpTransport.newTrustedTransport();
    final JsonFactory jsonFactory = JacksonFactory.getDefaultInstance();
    return new Gmail.Builder(transport, jsonFactory, createCredential(transport, jsonFactory))
        .setApplicationName("gmail-api-app")
        .build();
  }

}