Answer by Andrew for Sending Email in Android using JavaMail API without using the default/built-in app

For those who want to use JavaMail with Kotlin in 2020:

First: Add these dependencies to your build.gradle file (official JavaMail Maven Dependencies)

implementation 'com.sun.mail:android-mail:1.6.5'

implementation 'com.sun.mail:android-activation:1.6.5'

implementation "org.bouncycastle:bcmail-jdk15on:1.65"

implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.7"

implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.7"

BouncyCastle is for security reasons.

Second: Add these permissions to your AndroidManifest.xml

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

Third: When using SMTP, create a Config file

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"}

Fourth: Create your Mailer Object

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)