Once the email was iterated upon and the idea was proven (i.e. that users actually wanted it in the form that existed), it was handed back to me to automate properly, which in that case took about 7-10 days of dev work.
Or not, if it turned out the customers simply weren't interested. There were plenty of experiments like that where I created a checkbox or something and then we quietly removed it 2 weeks later.
It was a refreshing change from past jobs where I'd spent weeks working on a feature asked for by users that we were so sure would be a game changer only to see 1 or 2 people actually use it.
In other words, doing things that don't scale is not just useful for validating the market, it's arguably even more useful for validating one's specific implementation of a product, and also derisking the business model as a whole rather than having high capex initially.
Of course, you should try to validate assumptions as quickly and cheaply as possible. If "doing things that don't scale" means temporarily not automating things involved in validating assumptions, then fine. But don't manually do things that are not part of that calculus.