TLS

TLS (Transport Layer Security) allows you to secure the Simple Mail Transfer Protocol.

You probably want to let your proxy handle TLS, this guide is for TLS in tests.

TLSContext (SSLContext)

Kotlinmailer provides a utility function allowing you to easily create a new TLSContext (actually an SSLContext).

val tlsContext = TLSContext(
    File("path/to/keystore"), keyStorePassphrase = "passphrase",
    File("path/to/truststore"), trustStorePassphrase = "passphrase"
)

If you need a keystore and truststore for testing purposes, you can download both from the OpenJDK repository.

Secure the SMTP server

Use the setupTLS function inside an SMTP server builder.

smtpServer {
    setupTLS(tlsContext)
}

Currently, TLSv1.3 and TLSv1.2 are enabled by default, but you can change that (example below).

or if you need more options

setupTLS(
    tlsContext,
    requireTLS = true,
    protocolVersions = listOf(TLSVersions.TLS_1_3),
    requireClientAuth = true
) {
    // configure the SSLSocket to your liking
}

Secure the Mailer

Inside a mailer builder, you can set the transport strategy:

mailerBuilder {
    // STARTTLS
    withTransportStrategy(TransportStrategy.SMTP_TLS)
    // or complete TLS encryption
    withTransportStrategy(TransportStrategy.SMTPS)
}