How to create an Outlook rule to assign a category to emails from contacts that have a specific category assigned?

"As I'm afraid there is no way to do this, the easier would be to create several Contacts folders, one per category.

Create a new folder (CTRL+SHIFT+E), indicating it will contain ""Contact Items"". Name it with the name of the category, ""Contacts - Work"" for instance. Create as many as needed.

Create as many rules as you have contacts folders, to assign the correct category to contacts for the specific contact folder."