There are a lot of miscommunication between me and the author of this post. The selected snippets of messages posted on the article seem to put me in a bad light, however it's only one side of the story. The selected messages posted on the article are ones that are favorable to his argument. For example the author did not post a screenshot of me saying that I will be looking into this but went ahead to write an elaborate blog post immediately to put Sendy in an unfavourable light.
Bugs and issues with security has been and always will be the top priority with Sendy over the years. I agree the client side parameter 'subform' bypasses the reCAPTCHA and should be fixed. It is an oversight. And it will be fixed.
There are concrete things you can do to improve.
1) When someone reports a security issue to you, always always always thank them, even if you think that they're wrong, and are about to tell them. They're taking time that they don't need to take, with the goal to help you deal with a problem that is so much more yours than theirs. In other words they are demonstrating generosity, so thank them for that. If it turns out that they were wrong and that there is indeed no issue, well, no skin off your back; if it turns out that they are right, you'll be glad to still have them on your side rather than writing posts.
2) Be more curious. Instead of declaring "This is not a vulnerability or a security issue", as if the issue was closed, you can simply ask: "I'm not understanding why this is a vulnerability or a security issue. Can you explain and demonstrate a proof-of-concept attack?"
3) Consider getting advice from a security-minded person on such issues. It doesn't really take credentials. It just takes a certain kind of mindset that, in my experience, is not held by the majority of even very talented software developers.
I have to say, reading your response paragraph that starts with "if a human opens up his browser console to remove the subform parameter...", I recognized a very common feeling in me. Oh no, this person is just not getting security. Same facepalm reaction as the author. Having an API parameter that lets an (untrusted) client override a security measure isn't an "oversight", and more like a big design flaw. Kind of like if someone had a login API with a parameter called "pretend_password_is_correct" that let you sign in as anyone when set to true. If you're not seeing the issue when pointed out to you so clearly, it is really in your best interest to not make security decisions by yourself.
You shouldn't feel alone in this. Most developers that I've worked with, even top talent, cannot manage to put themselves in an attacker's shoes. Usually it's hard enough to put yourself in the normal user's shoes and get the thing to work for them. Following advice from people who have the skill of thinking like an attacker is the most valuable thing you can do to protect yourself.
I've been asking this hundreds of times but never got an answer, why doesn't Sendy add a visual e-mail builder?
Customers are going crazy over the issues with Wysiwyg and tables to make simple two-column designs, while drag/drop builders are straightforward and simple while keeping the code fully compatible with mailers.
Basically all competitors have drag/drop builders now, even the simpler one-time-pay scripts on Envato.
Sendy's pricing model simply doesn't allow such 3rd party product licensing and I imagine maintaining a heavy front-end application like a drag-and-drop template builder across the variety of clients is too costly (dev and support time) as well.