Relay-Only MTA?

I am trying to find a way to relay mail from a machine that is connected to the WAN via a dynamic non-public IP to a legitimate SMTP server in such a way that the email appears to originate from the legitimate SMTP server. I am not a spammer, I am not trying to perform anything illegal, I just want to be able to send mail from my machine and not have it bounce because it originated from a dynamic non-public IP. I know of some open-source solutions to this problem (like eSMTP) but would prefer to keep this within the context of my C# application. Does anyone know a way to do this?