Let's say I write a paper and use a result from someone that is rarely cited. Would it be polite/acceptable/unacceptable to email them and let them know of my application of their result before publishing my paper?

This would be primarily self-serving on my part, as I'd want to know if they see any other applications of their work to mine. But I don't want to come across as someone who's trying to leach ideas off of them. I'm open to collaboration.

What would be your reaction if you received an email saying something to the effect of: "Hey, I used result X from your paper from 10 years ago to show Y. Do you think other applications of your related work could apply to mine? Here are some of my questions, etc...".