Hi tgperez,
I received your PM notice by email, but I couldn’t figure out how to actually go and look at it and answer it directly, so I guess it’s better to just post on your thread again to “bump” it if you need further assistance.
As I said above, I don’t think you can put a captcha in a mailinglist form. At least it isn’t referenced in the Guide that you can do so. But maybe somebody else has figured out a way to make it happen. And the {__TextCaptchas} tag question I had for you is still unanswered.
Anyways, as to getting emails, that is the purpose of the mailinglist form, to collect email addresses. Are you saying that you aren’t getting what you want, or there is something wrong with them? When someone puts an email address in the mailinglist form field, they’ll be sent an email that they have to click on a link to verify. This weeds out robots and the like. Which is why I assume that EllisLab might assume that a captcha field isn’t necessary.
Of course, you could always use a third party captcha solution. Snaptcha is awesome, or so I’ve read. I’m going to try it soon as a captcha replacement.