You receive the following error when adding a new comment and sending an email. The comment is added but the email is not sent.
You have exceeded your organization's daily single (outbound) API/Apex email limit. You can email a maximum of 5,000 (increased from 1,000 in Winter '17) external email addresses (non-users) each day via the API/Apex regardless of your Salesforce edition. Consumption of this limit resets each day at mid-night GMT. Unfortunately this limit does not scale with user count like most SFDC limits. The single and mass email limits don't take unique addresses into account. For example, if you have firstname.lastname@example.org in your email 10 times, that counts as 10 against the limit.
Emails sent via the UI (Send an Email button), Workflow and Auto-Response Rules do not count against this limit. Conversely, inbound email limits do scale by 1,000 emails per Salesforce user license.
Contact your Salesforce.com administrator, Salesforce.com account representative, or Salesforce.com technical support to obtain an increase to this limit or for clarification on the limit. If you are unsuccessful in obtaining a suitable increase, please contact email@example.com and reference your Salesforce case number for the limit increase request.
For more information, see:
"Single email messages sent with the sendEmail method count against the sending organization's daily single email limit. When this limit is reached, calls to the sendEmail method using SingleEmailMessage are rejected, and the user receives a SINGLE_EMAIL_LIMIT_EXCEEDED error code. However, single emails sent through the application are allowed."
The "application" refers to emails sent using workflow or by clicking the "Send an Email" button.