Translation:I really do not want to send it to you.
I have a question about the position of 'really' in English, and how it translates into Italian. I REALLY don't want to send it = I am strongly against it. I don't REALLY want to send it = I have reservations, I am reluctant. There's quite a difference between these two - so which one does 'non te la voglio proprio mandare' cover? BTW - since it is a want/desire, I can see the argument for using the subj instead. :)
To say the sentence you ask for requires the present subjunctive. You can get a feel for this whenever you need to put a "that" = "che" in the sentence. "I really don't want that you send it." Peter almost gets it right but he follows the "che" with something else and is missing an "it" (but I am sure he would be understood.) "I really don't want you to send it" = "Davvero, non voglio che tu lo mandi"
I was taught that the subjunctive mood was meant to express uncertainty. So you would use it if you were saying something like "I doubt that I want you to send it", but never if you were saying something like "I am sure that I don't want you to send it." Am I mistaken?
Dunno how I managed to drop the "it" from that sentence. Oops.
You are absolutely right that the subjunctive is used to express uncertainty.(As well as thoughts and hopes.) The uncertain bit in your sentence "I am sure that I don't want you to send it" is whether or not it will be sent. It may help to think of your sentence as "I am sure that I don't want THAT you send it" This leads you to the subjunctive.