If you just want to build an email engine, that's super trivial to do in almost any programming language. It's like 20-100 lines of code, including pulling a file full of email addresses. The major problem with stuff like that is that they are noisy, they don't deliver batches in single connections (like all @hotmail.com is not delivered in one connection - not one email but one connection), etc... But the upside is that it's easier to spoof if you write it from scratch. Once you've compromised a machine the only trick is to ensure that they have whatever libraries you need to run your script.
And yes, there are many ways to hack into computers without getting caught. Look at http://www.milw0rm.com/ for ideas.
- RSnake
Gotta love it.
http://ha.ckers.org