How to open third party "non mapi" email client on windows?

Question

Being more specific, we have some legacy programs that our clients are using that allow exporting of an image or document from the application to outlook or their "default email client". We are currently writing an application that would capture this data and give them additional options to send this sensitive data using another application/service before they send sensitive data over outlook.

We were able to register this application to work in windows 10/7 (using the proper registry entries and file associations) for default handling of mailto .eml etc, but we are unable to have it open using "send mail" (right click send to mail recipient for example). The applications they are exporting from invoke MAPI (I believe?) calls, and we get the "No default email program registered" error.

All we need is our application to be opened using the default sendmail/mapi handler, we can capture the arguments and take it from there. We tried setting the "DLLPath" entry in the registry for our application, and it does not open it. Is there a way to do this? We just need windows to open our application using the default send mail!

After doing extensive googling, I have a vague understanding that we have to write a DLL just to open our application? This seems a little absurd, we just need the default behavior to just open the application with what ever arguments it pleases, we can interpret and go from there.

Thank you for your time, any direction would be appreciated.