Sounds like you were either hacked by a spammer or had a spam attack that filled up the queue with bounce mails to me.
You may want to give us some more details here as I'm assuming by WHM you mean cPanel's WebHost Manager you're talking about? Linux or Windows? If so, I believe that cPanel uses Exim's MTA [exim.org] and you would need to probably find out what command line tools are available for Exim and use those. Alternative, just find their mail queue file structure on the drive and simply look at a few files.
Sorry I haven't used cPanel or Exim much and can't help more, but they're all basically the same under the hood for the most part and shouldn't be too hard to figure it out.