Know-How Exchange : E-Marketing

I want to offer turnkey email marketing services to realtors. This involves finding an email marketing company (like Vertical Response or the like) who will open one account (for me) with multiple users (my agents). I will manage all of the users content and email delivery. My objective is to send out emails for multiple users at one time instead of going in and out of each agent's account. Each user's emails will be 'branded' with their headshot and company logo, etc.

The agents will be further broken down by company into groups of roughly twenty apiece so that the content won't overlap between local companies.

The only email marketing company I've talked to so far who understands what I'm trying to do is Topica. I called Vertical Response but no one called me back. Does anyone know of another email marketing company that can handle the sort of thing I'm talking about?? I'd like to compare Topica to a competitor.