How can I validate my email marketing idea?

I'm thinking of doing my own version of an email marketing software. I don't want to compete with big players so I'm thinking of a specific niche. So my question is, as far as building an MVP is concerned, how did you validate your Ideas? was it simply a sign-up form? If so how did you communicate your idea? or did you build out part the project just demonstrate is the capability? Just looking for information on different ways that I can validate whether my idea will work or not.