EmailService.kt

//build.gradle (:app)
//implementation 'com.sun.mail:android-mail:1.6.2'
//implementation 'com.sun.mail:android-activation:1.6.2'

import io.reactivex.Completable
import io.reactivex.disposables.Disposable
import io.reactivex.schedulers.Schedulers
import java.lang.Exception
import java.util.*
import javax.mail.*
import javax.mail.internet.InternetAddress
import javax.mail.internet.MimeMessage

object EmailService {
    val email = "your.email@gmail.com"
    val password = "password"

    fun sendMail(recipient: String, subject: String, message: String): Disposable {
        return Completable.create { emitter ->

            val properties = Properties().also {
                it["mail.smtp.host"] = "smtp.gmail.com"
                it["mail.smtp.socketFactory.port"] = "465"
                it["mail.smtp.socketFactory.class"] = "javax.net.ssl.SSLSocketFactory"
                it["mail.smtp.auth"] = "true"
                it["mail.smtp.port"] = "465"
            }

            val session = Session.getDefaultInstance(properties, object : Authenticator() {
                override fun getPasswordAuthentication(): PasswordAuthentication {
                    return PasswordAuthentication(email, password)
                }
            })

            try {
                MimeMessage(session).let { mime ->
                    mime.setFrom(InternetAddress(email))
                    mime.addRecipient(Message.RecipientType.TO, InternetAddress(recipient))
                    mime.subject = subject
                    mime.setText(message)
                    Transport.send(mime)
                }
            } catch (e: Exception) {
                Log.d("RYCHU", "SendEmailService sendEmail() Catch ${e.message}")
                emitter.onError(e)
            }
        }.subscribeOn(Schedulers.io())
            .subscribe({
            }, {
                Log.d("RYCHU", "SendEmailService sendEmail() Error ${it.message}")
            })
    }
}