Free for up to X Users per Domain Model

Currently my offerings use a simple per month subscription model. But this is making it hard to stir up interest in the product in an organic way. I was thinking about offering my product for free, but limiting the free subscriptions to about 3 users per domain (with the exception of common email domains e.g. GMail ). I think this would have the effect of allowing most people get a copy for themselves, but getting large companies that have many users to foot the bill (They seem like the most likely to pay for software anyway).

Has anyone tried this model? I am curious to hear if you have had any success / what are the pitfalls?