I had planned on using an open source SMTP server as a base and adding new features in to do whatever parsing I need and then send the response but I'm wondering if there is an even easier way to do this.
Is rolling my own SMTP server my best option or is there an easier way for me to do this?
I would prefer to use .NET to do this.
Rolling your own SMTP server is an interesting challenge but not trivial. There are a number of Request for Comments (RFCs) that specify what an SMTP server needs to handle. Have a look at the Wikipedia article for some info on what RFCs apply, in particular RFC 5321.
Unless you have a lot of time on your hands or only need to implement a very small subset of the features I'd suggest using an open source server. There are some sample projects out there (e.g. CodeProject) that might be a good starting point if you want to build your own.
Accomplishing this with the built-in .NET libraries is relatively simple.
The place to start would be to write a program to download the mail: here's a great example using POP3/C#.
Once you have that you can parse/store the e-mail contents using regex or whatever rules engine you're building and then use .NET smtp libaries to send out your responses.
It's very easy to send emails in .Net via SMTP, downloading and parsing them is a little bit more difficult but there's quite a few articles about writing POP3 clients so it's definitely doable. Or if you're willing to spend some money there are quite a few POP3 libraries to buy, but I've never tried any so I can't give any recommendations.
I don't think you want to deal with writing your own SMTP server because the conversation between the client and the server can be inconsistent and complex. Most SMTP servers let you specify a MDA (Mail Delivery Agent), like procmail, to deliver mail destined for users on that machine.
You can easily write procmail rules to pass emails through your program, which has the advantage of allowing you to call different scripts based on the content of the email headers and bodies, or write your script as an MDA itself.
Rolling your own SMTP server would be pretty much crazy. Why not connect to an existing SMTP server? Presumably the incoming email is coming from a server that also supports outgoing email.