For which recipients is the calendar available?

The email-integrated calendar is available for more than 2.1 billion active users: if your lead opens your email on Gmail, G Suite, Outlook, Yahoo, Hotmail, MSN or Mail.ru, your calendar will display inside the email. 

If your prospect doesn’t open your email on Gmail, G Suite, Outlook, Yahoo, Hotmail or MSN, your calendar will not be inside the email. Instead, the email will automatically have a button with a link.
The link redirects him to a branded page with your updated calendar and availability, to book an appointment.