Using Send Email (emailauthor.jsp) To Get Around Low Single Email Limits
You receive the following error when adding a new comment and sending an email. The comment is added but the email is not sent.
Sending e-mail with new comment failed: SendEmail failed. First exception on row 0; first error: SINGLE_EMAIL_LIMIT_EXCEEDED, null: 
These are known Force.com platform limits that are difficult to work around.