Keep in mind, if this email is real, it's probably not just some empty threat. Guzman's Sinaloa Cartel has been connected to crimes just as horrendous as those committed by ISIS, including mass killings and beheadings. Some experts even say ISIS is copying the Mexican drug cartels to a T.
Guzman has been evading capture since he escaped from a Mexican prison via a tunnel over the summer.
El Chapo Reportedly Declares War on ISIS | Complex