Do not remove your only MTA just yet.
Easiest way to install qmail(it has a crippled lisence that prevents it to be distributed as a binary <Moral statement> You should not use it.</Moral statement>), is to
- install debian package called qmail src. (by command "apt-get install qmail-src").
- Run command "build-qmail" and follow the instructions on the screen. It do gives you a change to apply your own patches and do changes to the source.
- Install the package that gets generated
This version of qmail has its filesystem structure adjusted for debian and some other debian spesific tricks put in place. As for an answer to your original question: In case someone have not prepackaged such a source package as in qmail case, the dependencies can be circumvented by using equivs package. see Apt-howto