Okay can u just clarify what you want to do.
You want to be able to offer ppl an dikkop.com email adddress automatically, upon say registeration or other.
You then want them to be able to access the email account via a weblogin, for reading,sending, storing.
Is this do able Yes it is.
How
With a degree of difficulty, and rather lot of setup i would imagine
Never done it myself but the first option i say is check with your host about mailing options, some may have a facility to offer it it. I will see if i can find any scripts for you but for such a complex script i donnt think u would get a decent free one, really do think the best route would be going done your hosting setting see whether they offer something like that