Download Java Script from this location.
Settings>Customization>Customize the System
From the left pane select Web Resources and click New to create new Web Resource.
Display Name: Email From Case
Type: Script (Jscript)
You can either use the Text Editor to copy and paste the JS or upload from file
Now add the JS Web Resource to the Email Entity, and trigger the script on load (when you start new Email).
From left pane extend the Entities node. Scroll down to the Email entity>Forms and select Email main form.
Click: Form Properties
Click Add to add the “Email From Case” web resource to the Form Libraries section.
In the Event Handlers section click Add>Select EmailFromCase and enter the function name: EmailFromCase in the Function fields.
Click OK, Save & Publish All Customization
Testing the solution
GoTo Settings>Service>Cases and open a case
Click: Add Task and select Email
The subject line of the Email includes the Title of the case plus case number