Email sending is done through \App\EmailQueue and you can see the outgoing and sent emails in Tools -> Email Queue. Emails are queued for data retention and compliance requirement. System emails are automatically placed in the queue to prevent risk of losing emails due to SMTP errors.
The system sheduler is configured to send emails out every minute (you can change this). If there is an SMTP failure, outgoing emails will remain in the queue so that you can manually send it out later. You can also manually set the status to inactive in case you are troubleshooting your SMTP server.
Note: email data is stored in both the database as well as in the file system (/web/your-site-name/data/private/mail) and is organized by year/month/date.
If you want to stop the email queue for maintenance, simply click that green checkmark and set it to Inactive.