Good question!  It really depends on the quality of the list.  I have had numerous clients run a postal, telemarketing, and email campaign from the same vendor, and typically were not satisfied with the email outcome.  The issue is typically how the data was pulled.  If it was pulled from a survey or from a trade show, then quite often it is not current.  This is why I tend to prefer to go to one vendor for postal and phone data and to another vendor for opt-in permission based email because then you are dealing with vendors who specialize; and the data in their specific niches tends to be superior.  The other issue, is that with opt-in email, it is truly beneficial to run more than one broadcast.   I typically recommend at least 3 spread out over a period of 2 to 3 weeks.