This one comes up over and over again in business e-mails. It is not translated by “thank you for your reactivity”.
I received an e-mail the other day from someone and here is the text. Here’s how to do it:
Thanks for getting back to me so quickly.
All my days are very busy at the moment but I’m free over lunchtime on Fri between am & pm lessons. Would 12.15 at my office in the Lyon be OK for you? Also, could you email me a copy of your CV?
No, you can’t say “reactivity”.