It is in server/default/jboss.jar.
When running the mail tests what I find easiest is to just set the JBOSS_HOME variable to your JBoss distribution, that way I only have dnsjava-1.5.0.jar in the jboss-mail/lib directory.
Here's a copy of my build.bat file:
REM convenience bat file to build with
REM ATTENTION: Set ANT_HOME to the root directory of ANT distribution