I would say "why are you sending me that?" I think this is the correct translation. Or, if it refers to something in the future "why will you send me that?"
The literal translation would likely be "for what purpose do you send me that?" However, when you use the present (envias) we often will not use the same verb form, but we will use the gerund (verb+ing) to make it correct. For example, you can say "corro (ahora)" but we have to say "I am running (now)" because "I run now" makes sense but is not correct. Another common example is "me voy" which, in English, you have to say "I am going" and not simply "I go."
Also, most Spanish speakers use the present form for something in the near future like "para que me envias eso mañana?" To translate that into the future you have to say "why will you send me that tomorrow?"