Now my question is: if I reply to wordpress@leandroperez.com.ar, is that person going to get my email?
I'm afraid not. No one will get an email sent to that email address, unless you set up an email address for that username.
There, in the "From" information sometimes I get an email with the format Sender Name <wordpress@leandroperez.com.ar> even though the sender has a different email address in the body of the message.
Jetpack's Contact form should indeed prefill the replyto field with the email of the person who submitted the contact form.
Could you copy the contact form shortcode you've added to your page, and paste it here so I can take a closer look adn see if I can reproduce the issue on a test blog of mine?
Thanks!