All recipients in the to, cc and bcc fields count against this limit for email sent using Apex or the API. An example being an email with 2 addresses in the “to” field and 1 address in the “cc” field would count as 3 calls.

1. Emails sent using setTargetObjectId() set against the user object do not count against the SingleEmailMessage limit.

2. Send Email” button on contacts.

3. “Send Email” button on Leads.

https://help.salesforce.com/articleView?id=000002490&language=en_US&type=1