This is an excellent concept that targets a real pain-point I faced many times, which is: How to properly lay-out or template emails without the pain of actually sending transactional emails through SMTP and looking at them in the client (Gmail, mobile, etc...). This also prevents designers and programmers to collaborate, so your concept would allow designers and marketeers to design and change email layouts independently from the programmer, which is another pain-point for non-developers.

Having said that, it seems that you expect the app to use your SMTP, and IMHO that's a deal-breaker. Once an app or company is already using a SMTP provider, changing to an unknown one (which could impact deliverability and so on) is a deal-breaker. However, if you provide a tool that somehow the marketing dept can change templates which then the developers can somehow fetch into the source codebase, now that would be (I think) a good value.

I am a software developer (not a designer nor marketeer) so I may not be the target user for this tool, but I think it would be valuable for developers too.