I'm looking to build a social networking site that notifies followers of a particular user when new content has been posted by that user. Since the hope is that the site will eventually have thousands of users with thousands of followers, I'd prefer to use an outside product/service to send the emails rather than building the functionality into the application and dealing with scalability issues.

I've looked at MailChimp and a few others out there. Since my use case isn't really email campaigns, these providers don't seem like the right solution. In fact, MailChimp support confirmed for me that my scenario doesn't fit for them.

I'm sure others have run into this same requirement. What are others out there using? I'm using .NET so would like to target something that has a .NET wrapper if possible.

Any advice would be most appreciated!