Season pork butt generously with dry rub seasoning. Place pork in a resealable plastic bag. Spread mustard evenly over butt while inside the bag to avoid mess. Seal bag and refrigerate overnight.
Preheat smoker to 225°F.
Place butt on middle rack of smoker and smoke for 7 to 8 hours or until internal temperature reaches 185°F. Remove from smoker. Cover with heavy-duty aluminum foil and return to smoker. Increase smoker temperature to 275°F. Smoke for an additional 2 hours or until internal temperature reaches 200°F. Let meat rest inside foil for 30 minutes. Then pull pork.