jSMTP

A good Java implementation of SMTP and extended SMTP.

Maven

jSMTP is using slf4j, so Maven is the best way to include jSMTP:

<repositories>
    <repository>
        <id>hellsh</id>
        <url>https://mvn2.hell.sh</url>
    </repository>
</repositories>
<dependencies>
    <dependency>
        <groupId>sh.hell</groupId>
        <artifactId>jsmtp</artifactId>
        <version>[1.1.0,2.0.0)</version>
    </dependency>
</dependencies>

Certificates

Using the power of openssl & keytools, you can convert a certificate.crt + private.key into a .jsmtp_keystore using password 123456 which you can then use as follows:

new SMTPServer(mySMTPEventHandler, ".jsmtp_keystore", "123456");