The bent-over rear delt flye is the most common exercise used to target the rear delts, but it suffers from some fundamental flaws. Why? Because the strength curve of the delts in isolation exercises is bell–shaped. This means you're strongest in the mid-range and weaker at the start and end ranges.
The dumbbell rear delt flye has a resistance profile where the load is heaviest at the top and lightest at the bottom. This doesn't match up with the muscle's profile and, therefore, means you're not fully challenging it throughout the lift.
The rear delt machine can help to improve the strength curve somewhat. Unfortunately, many gyms don't have this piece of equipment, leaving you reduced to making sub-par gains with the dumbbell version.
But all is not lost. You can do a superior version using a bench, a dumbbell, and an adjusted body position.
By lying sideways on the bench you can create a resistance profile where the lift closely matches the muscle's capacity. Because of your position, the lever arm is very small to begin with; it increases as you move through the mid-range, before dropping off at the end. This is ideal and means the rear delts are challenged through every inch of every rep.