To get the idea of how the app works watch this short video tutorial. Mail Terrier is free for home users and very affordable for commercial use.
Quick answer: To search emails with Mail Terrier, install the program, point it at the folder that holds your message files (PST, EML, MSG, MBOX), type your keywords into the search box, and press Search. Matching messages are listed instantly and you can open or preview each one. It runs offline, indexes whole folders, and is free for home use.
Download free trial and convert your files in minutes.
No credit card or email required.