Can't apply rule to existing emails

I have a rule (only one!) that works on incoming emails. However, when I want to apply it to existing emails, the button is greyed out. How do I apply a rule to existing emails?

To manually apply a rule, see the Gary’s post below from the following thread.

“How to run a rule manually”

“To apply a Rule manually, right-click on your Inbox and choose Apply Rule”.

“You can then select the Folders and Rules you want to apply in that instance”.

Also in the rules window (at the top) in the dropdown you can select from “local and server” rules. So check if your rule you setup is showing in the Rule window. If not change it to the opposite one and try again manually running it.