To send emails from the application, you must have configure the mail driver.
The log driver doesn't send any email. It only logs the email in storage/logs/laravel.log file
To use the Mailgun driver, you must have valid & active mailgun account. You need to add your domain in your mailgun account & activate it. Once activated, you can use your domain, secret key & mailgun end point to send emails.
To use the SMTP driver, you must have a valid SMTP carrier. If you use Gmail SMTP, please make sure you have disabled less secure app option in your Gmail account.