Those who mock the ones preaching do so because they are fiercely effected, or else why would they bother to do so? They would have never bothered if they were not effected, they would have gone about their own business. They mock merely to calm themselves: That is because their good conscience starts disturbing them. That is a precaution taken by unbelievers against that disturbance.
If you define the reward you get while preaching religion as three in numbers, it becomes thirty when you do it after being mocked. If the severity of mocking is high, it would be three hundred. If it is very very severe, it would be three thousand, let me say. What is your goal? Is it not an act of worship? God creates an act of worship for you.
They say, "We are preaching Islam, but they mock us." Who is the One that makes them mock? God is the One Who does that. He creates it at the same time with the preaching. It is a hidden honor created by God to increase the worth of preaching, of that act of worship. It is against unbelief, it is in favor of Muslims. It is an honor for Muslims and a scourge on disbelief. When you do not take that mocking seriously, your reward would increase many fold.