Draft emails feature in Salesforce allow the customer support agents who use the case feed to write and save messages without having to send them immediately. 

Reference Article – https://help.salesforce.com/articleView?id=sf.case_interaction_enabling_email_drafts.htm&type=5

1.   Enable “Email-to-Case” in Salesforce Setup.

2. Enable Email Drafts in Support Settings.

Output:

SOQL to query:

SELECT Id, Subject, TextBody 
FROM EmailMessage 
WHERE Status = '5'