She certainly doesn't look to be purebred.
It's hard to tell her color. Is it brindle (fawn w/ black stripes) or is it black? It also looks as though her legs have a white, brown and black portion? Boxers only come in 3 colors...brindle (fawn with stripes), fawn and white.
And yes! her muzzle is not a typical boxer length (but as you can see in many pictures throughout the gallery the boxer muzzle varies usually depending on breeding practices). She may be a boxer mix, as Mckenzine said just look for the boxer traits

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What ever she is she is really cute lovicon
And even though she may not be a boxer this website can certainly help you with any puppy problems/questions you may have.
Have Fun with the new puppy.