Related Post filter by category

Dear sir/madam,

I am having a problem with the related post plugin. The related post plugin is active on each post on our website, but it is also searching for related posts outside the assigned category. What I would like to know is how I can let related post plugin only look inside the category's the post has assigned.

Can you please help me on this matter?

Thank you very much in advance

Yours sincerely,

Daniël Pietersen

Newsite BV