Email API v.2 - Subject property Explained

Subject is a short summary of the email content.
Although it may be empty, it is better to have it in place, since most of the email client will complain and consider that message as possible spam.
Subject is a string property. For compatibility reasons it is restricted to 144 characters in UTF-8 encoding.

subject is a string that contains a short summary of message contents

subjectStringSubject of the message. No more than 144 characters in UTF8 encoding