The Automatic-Module-Name org.simplejavamail.java-utils-mail-dkim from the Manifest.MF is not a valid Java module name. This causes this library to be unusable from within a java module, since this is treated as a syntax error:

module mymodule {
    requires org.simplejavamail.java-utils-mail-dkim;
}

There is already a PR to fix this: #5