[analog]

Roundcube would probably do the job. The problem is what if the guy says "but this doesn't have feature X that gmail has, add it or I won't pay you."

If you could change the spec to be "install roundcube" then maybe it wouldn't be such a bad deal, but then you're essentially doing the most important part of the work (telling him how to get what he wants) for free.

[hippich]

And this will happen. Usually such projects get half-done and half-paid.