DKIM support for spring-boot-starter-mail.

Avoid your sent email in spam folder.

• DKIMSigner to sign MimeMessage
• AOP @Before to interceptor JavaMailSender.send and sign MimeMessage before sending

• Add following dependency in your pom.xml:

<dependency>
    <groupId>org.mvnsearch.boot</groupId>
    <artifactId>dkim-spring-boot-starter</artifactId>
    <version>1.0.0-SNAPSHOT</version>
</dependency>

• in application.properties, add following configuration

dkim.signing-domain=demo.com
dkim.selector=default
dkim.privateKey=classpath:/rsa/demo.private.key.der

• User JavamailSender as usual because of AOP

a DKIM key setup:

There are test keys in the src/test/resources/rsa/ directory but be aware to use those only for your tests.

You need:

• a private key on your hard disc (e.g. in keys/); you can generate a new key by

     openssl genrsa -out private.key.pem

• DKIM for JavaMail needs the private key in DER format, you can transform a PEM key with openssl:

     openssl pkcs8 -topk8 -nocrypt -in private.key.pem -out private.key.der -outform der

• a public key in your DNS; here is a sample resource record with selector "default": default._domainkey IN TXT "v=DKIM1; g=*; k=rsa; p=MIG...the_public_key_here...AQAB" (see https://www.ietf.org/rfc/rfc4871.txt for details) You can use openssl to get a public key from the private key:

      openssl rsa -inform PEM -in private.key.pem -pubout

• Mail Tester: https://www.mail-tester.com/
• DKIM: https://www.dkim.org/
• 28 Tips To Avoid Spam Filters When Doing Email Marketing: https://monk.webengage.com/how-to-avoid-spam-filters-when-sending-emails/
• DKIM for Java: https://github.com/globalbus/dkim
• Configuring and Managing SPF, DKIM, and DMARC: https://app.pluralsight.com/library/courses/configuring-managing-spf-dkim-dmarc/table-of-contents