Yup, as jpmoriarty said, the only chance you have is if its a dedicated IP, and even then its still hard. With some ISPs, you can look at the hostname associated with the IP, for example:
212.150.15.250
jsmith.ispprovider.com
In the example above, you could try jsmith@ispprovider.com and you have a reasonable chance of getting it right. If the hostname comprises of sets of digits or dots, its un-emailable 🙂. Hope that helped.
Luke