Bind SMTP to IP Address

We have a Windows Server 2003 installation with multiple SMTP Virtual Servers (using IIS, not Exchange). Each SMTP Virtual Server is bound to a unique public IP Address. This system works just fine for incoming mail. However, our problem is that for outgoing mail, the IIS / Windows uses ONLY the primary IP Address that is associated with the NIC (this is a known limitation). We need someone to develop a solution to get around this limitation and force each SMTP Virtual Server to use the unique IP Address for Outbound mail as well as incoming. Some possible solutions that we have considered: - SMTP Event Sink - SMTP Proxy - Third party software - Custom Development (Any code should be in C#.NET) Please provide a brief synopsis of what you would do to solve the problem, as well as your timeline to complete it. Please do your own research before bidding. This issue does not have a 'cut & paste' solution, so we need someone with some common sense and expertise.