I want my marketing guy to be able to define emails that will be sent to my users based on queries that he should be able to build.
As a developer I just want to send what a specific user do to this automation app and let the marketer deal with email automation:
Infusionsoft would be another option. It's meant for businesses already with a steady flow of prospects and customers and are slightly beyond the "startup" and "idea" stage. It's the most powerful marketing automation software for small business and email is one of its strong suits.
For larger applications a combination between Salesforce.com and ExactTarget would work, too. But for most small businesses, Infusionsoft is an all-in-one marketing suite for this.
See the details on what they provide:
If you've worked hard to get the permission of users to market to them directly, I wouldn't take the value of that list lightly. For starters, I would highly suggest outsourcing it to a email marketing service provider - because the simple application you're talking about would easily be out done by one of these services.
Quick Google search pulled up this 2011 Best Email Marketing Service Comparisons and Reviews - which I'd take with as a starting point for features and providers, not an authority who to select.
When I was looking for an email list management application, I took a liking to MailList King out of New Zealand. It's a powerful app from a small company that provides good support. You might check with them whether they can customize any functionality you need that they don't already offer.
Since your user data is likely in your database. If you important it into (or better, sync it with) say Outlook then you could likely write a simple VBA routine to do it for you.
It seems it would take a very short time to write such a utility (a bit small to be called an app). You could likely get someone on one of the freelance sites (http://vworker.com or http://guru.com or http://elance.com ) to write it for you for less than $100.