为了帮助那些获得网络主线程异常与SDK目标>9。这是使用droopie的代码上面，但将类似于任何工作。

StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();

StrictMode.setThreadPolicy(policy); 

android.os.NetworkOnMainThreadException

您可以如下所示使用AsyncTask

public void onClickMail(View view) {
    new SendEmailAsyncTask().execute();
}

class SendEmailAsyncTask extends AsyncTask <Void, Void, Boolean> {
    Mail m = new Mail("from@gmail.com", "my password");

    public SendEmailAsyncTask() {
        if (BuildConfig.DEBUG) Log.v(SendEmailAsyncTask.class.getName(), "SendEmailAsyncTask()");
        String[] toArr = { "to mail@gmail.com"};
        m.setTo(toArr);
        m.setFrom("from mail@gmail.com");
        m.setSubject("Email from Android");
        m.setBody("body.");
    }

    @Override
    protected Boolean doInBackground(Void... params) {
        if (BuildConfig.DEBUG) Log.v(SendEmailAsyncTask.class.getName(), "doInBackground()");
        try {
            m.send();
            return true;
        } catch (AuthenticationFailedException e) {
            Log.e(SendEmailAsyncTask.class.getName(), "Bad account details");
            e.printStackTrace();
            return false;
        } catch (MessagingException e) {
            Log.e(SendEmailAsyncTask.class.getName(), m.getTo(null) + "failed");
            e.printStackTrace();
            return false;
        } catch (Exception e) {
            e.printStackTrace();
            return false;
        }
    }

那些正在获得ClassDefNotFoundError的人尝试移动这三个jar文件到你的项目的lib文件夹，它为我工作!!

如果你被要求保持jar库尽可能小，你可以分别包含SMTP/POP3/IMAP功能，以避免“dex中有太多方法”的问题。

您可以在javanet网页上选择需要的jar库，例如mailapi.jar + IMAP .jar可以让您在IMAP协议下访问icloud、hotmail邮件服务器。(在additional.jar和activate .jar的帮助下)

对于那些想在2020年与Kotlin一起使用JavaMail的人:

首先:将这些依赖项添加到构建中。gradle文件(官方JavaMail Maven依赖项)

实现“com.sun.mail: android-mail: 1.6.5” 实现“com.sun.mail: android激活:1.6.5”

实现“org.bouncycastle: bcmail-jdk15on: 1.65” 实现“org.jetbrains.kotlinx: kotlinx-coroutines-core: 1.3.7” 实现“org.jetbrains.kotlinx: kotlinx-coroutines-android: 1.3.7”

BouncyCastle是出于安全考虑。

第二步:将这些权限添加到AndroidManifest.xml中

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

第三:当使用SMTP时，创建一个配置文件

object Config {
    const val EMAIL_FROM = "You_Sender_Email@email.com"
    const val PASS_FROM = "Your_Sender_Password"

    const val EMAIL_TO = "Your_Destination_Email@email.com"
}

第四:创建Mailer对象

object Mailer {

init {
    Security.addProvider(BouncyCastleProvider())
}

private fun props(): Properties = Properties().also {
        // Smtp server
        it["mail.smtp.host"] = "smtp.gmail.com"
        // Change when necessary
        it["mail.smtp.auth"] = "true"
        it["mail.smtp.port"] = "465"
        // Easy and fast way to enable ssl in JavaMail
        it["mail.smtp.ssl.enable"] = true
    }

// Dont ever use "getDefaultInstance" like other examples do!
private fun session(emailFrom: String, emailPass: String): Session = Session.getInstance(props(), object : Authenticator() {
    override fun getPasswordAuthentication(): PasswordAuthentication {
        return PasswordAuthentication(emailFrom, emailPass)
    }
})

private fun builtMessage(firstName: String, surName: String): String {
    return """
            <b>Name:</b> $firstName  <br/>
            <b>Surname:</b> $surName <br/>
        """.trimIndent()
}

private fun builtSubject(issue: String, firstName: String, surName: String):String {
    return """
            $issue | $firstName, $surName
        """.trimIndent()
}

private fun sendMessageTo(emailFrom: String, session: Session, message: String, subject: String) {
    try {
        MimeMessage(session).let { mime ->
            mime.setFrom(InternetAddress(emailFrom))
            // Adding receiver
            mime.addRecipient(Message.RecipientType.TO, InternetAddress(Config.EMAIL_TO))
            // Adding subject
            mime.subject = subject
            // Adding message
            mime.setText(message)
            // Set Content of Message to Html if needed
            mime.setContent(message, "text/html")
            // send mail
            Transport.send(mime)
        }

    } catch (e: MessagingException) {
        Log.e("","") // Or use timber, it really doesn't matter
    }
}

fun sendMail(firstName: String, surName: String) {
        // Open a session
        val session = session(Config.EMAIL_FROM, Config.PASSWORD_FROM)

        // Create a message
        val message = builtMessage(firstName, surName)

        // Create subject
        val subject = builtSubject(firstName, surName)

        // Send Email
        CoroutineScope(Dispatchers.IO).launch { sendMessageTo(Config.EMAIL_FROM, session, message, subject) }
}

Note

If you want a more secure way to send your email (and you want a more secure way!), use http as mentioned in the solutions before (I will maybe add it later in this answer) You have to properly check, if the users phone has internet access, otherwise the app will crash. When using gmail, enable "less secure apps" (this will not work, when you gmail has two factors enabled) https://myaccount.google.com/lesssecureapps?pli=1 Some credits belong to: https://medium.com/@chetan.garg36/android-send-mails-not-intent-642d2a71d2ee (he used RxJava for his solution)

您可以使用JavaMail API来处理电子邮件任务。JavaMail API可以在JavaEE包中获得，它的jar可以下载。遗憾的是，它不能直接在Android应用程序中使用，因为它使用的AWT组件在Android中完全不兼容。

你可以在以下位置找到JavaMail的Android端口: http://code.google.com/p/javamail-android/

将jar添加到应用程序并使用SMTP方法

我为其他需要帮助的人找到了一个更短的选择。代码是:

package com.example.mail;

import java.util.Properties;

import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;

public class SendMailTLS {

    public static void main(String[] args) {

        final String username = "username@gmail.com";
        final String password = "password";

        Properties props = new Properties();
        props.put("mail.smtp.auth", "true");
        props.put("mail.smtp.starttls.enable", "true");
        props.put("mail.smtp.host", "smtp.gmail.com");
        props.put("mail.smtp.port", "587");

        Session session = Session.getInstance(props,
          new javax.mail.Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(username, password);
            }
          });

        try {

            Message message = new MimeMessage(session);
            message.setFrom(new InternetAddress("from-email@gmail.com"));
            message.setRecipients(Message.RecipientType.TO,
                InternetAddress.parse("to-email@gmail.com"));
            message.setSubject("Testing Subject");
            message.setText("Dear Mail Crawler,"
                + "\n\n No spam to my email, please!");

            Transport.send(message);

            System.out.println("Done");

        } catch (MessagingException e) {
            throw new RuntimeException(e);
        }
    }
}

来源:通过JavaMail API发送电子邮件

希望这能有所帮助!好运！