Mailbombing is just lame, plain and simple. Why? anybody can do it, there is no knowledge required. AND you will get caught since all the programs claim to be anonymous wich they are not.

Altough if u don't want to get caught (here comes the knowledge part) use an anonymous remailer(if you know how to write the script). If you are on ADSL and the target starts bitching and moaning to your ISP's abuse department it could lead to downtime for you.