Ask HN: How to validate an idea in the email plugin space?

Large companies in the space I'm investigating are using the benchmark (and expensive) enterprise solution, but medium and small companies aren't using anything at all.

I believe that some automation or crm plugins can be customized to do what I have in mind, but it wouldn't be as fluid.

3 small-medium companies have already given me the thumbs up to build them a demo, but I'm not sure if I should invest in an alpha or not. I've shown them mockups but it's really hard to get the idea across without showing an interactive dummy version of the app. I'd just like to not fall into the trap I fell into before where I sink lots of time and money into a beta just for everyone to change their minds and not opt in at the last second.