Sending email in Kotlin: libraries, examples, and SMTP setup

Sending email in Kotlin: overview of tools and code examples

Sending emails is a common task in web development, notifications, and background services. In the Kotlin ecosystem running on the JVM, you can use both classic Java libraries and modern Kotlin-oriented solutions. In this article, we will cover three main approaches: javax.mail (Jakarta Mail), Ktor Client, and Apache Commons Email. You will learn how to configure an SMTP client, send a simple email, and add attachments.

1. javax.mail (Jakarta Mail) — the classic standard

The most common library for working with email in Java and Kotlin. It fully supports SMTP, IMAP, and POP3 protocols. For a Kotlin project, simply add the dependency to build.gradle.kts:

dependencies {    implementation("com.sun.mail:javax.mail:1.6.2")}

Example of sending a simple text email:

import javax.mail.*import javax.mail.internet.InternetAddressimport javax.mail.internet.MimeMessageimport java.util.Properties

fun sendSimpleEmail() {    val props = Properties().apply {        put("mail.smtp.auth", "true")        put("mail.smtp.starttls.enable", "true")        put("mail.smtp.host", "smtp.gmail.com")        put("mail.smtp.port", "587")    }

    val session = Session.getInstance(props, object : Authenticator() {        override fun getPasswordAuthentication(): PasswordAuthentication {            return PasswordAuthentication("your-email@gmail.com", "your-password")        }    })

    val message = MimeMessage(session).apply {        setFrom(InternetAddress("from-email@gmail.com"))        setRecipients(Message.RecipientType.TO, InternetAddress.parse("to-email@example.com"))        subject = "Hello from Kotlin!"        setText("This is a test email sent using javax.mail.")    }

    Transport.send(message)    println("Email sent successfully!")}

Important: for Gmail you will need an app password or OAuth2. Do not store credentials in the code — use environment variables.

Sending an email with an attachment

import javax.activation.DataHandlerimport javax.activation.FileDataSourceimport javax.mail.internet.MimeBodyPartimport javax.mail.internet.MimeMultipart

fun sendEmailWithAttachment() {    // ... session setup as above ...    val message = MimeMessage(session).apply {        setFrom(InternetAddress("from@gmail.com"))        setRecipients(Message.RecipientType.TO, InternetAddress.parse("to@example.com"))        subject = "Email with attachment"    }

    val multipart = MimeMultipart()

    // Text part    val textPart = MimeBodyPart().apply {        setText("See the attachment.")    }    multipart.addBodyPart(textPart)

    // Attachment    val attachmentPart = MimeBodyPart().apply {        dataHandler = DataHandler(FileDataSource("/path/to/file.pdf"))        fileName = "document.pdf"    }    multipart.addBodyPart(attachmentPart)

    message.setContent(multipart)    Transport.send(message)}

2. Ktor Client — a modern asynchronous approach

If your project already uses Ktor for HTTP requests, you can send email via SMTP using the ktor-client-smtp plugin. This provides the benefits of coroutines and asynchronous execution. Dependency:

dependencies {    implementation("io.ktor:ktor-client-core:2.3.0")    implementation("io.ktor:ktor-client-smtp:2.3.0")}

Example of sending:

import io.ktor.client.*import io.ktor.client.smtp.*import io.ktor.client.request.*import io.ktor.client.statement.*

suspend fun sendViaKtor() {    val client = HttpClient {