Ideas cannot be copyrighted, so, yes, it is legal to use someone else's idea.
If the person who emailed you with a suggestion has fleshed out his idea with a plot line and characters and action scenes, and then registered this developed idea with the Copyright Office, THEN you could have trouble using the suggestion made. Otherwise, as long as you take the suggestion and make it your own, there should be no problem.
Although, I should add, and as you know by now, breakaway, from being on this forum, anyone can sue for any reason whatsoever, even if they have no chance of winning a suit, so there is
always a risk of that, however slight it might be.