Rules can't run retroactively, so for past messages we recommend using our search filters to grab a set of conversations, then use mass action to apply a tag. 

Here's more info on how to do this: 

Step 1: Used advanced search filters to pull all conversations with a certain keyword/keywords. We have an article here in our Help Center with a breakdown of those search filters and how they work. 

Step 2: Once you've got the emails narrowed down by search, you can apply a tag en masse using a mass action in Front. Here is an article with instructions on how to apply a tag en masse. 

From there, all the historical conversations will be tagged. Future conversations will be tagged by the rule.