You have to set up SMTP in order to use this feature.
Set up SMTP here https://vyper.io/user-settings/smtp
You can do it by going to the Launch tab and then Email Broadcast:
Only participants who have confirmed their emails will receive the email blast sent from Vyper. This is due to potential spam issues.
Note: if you'd like to send an email to ALL participants, you can manually export your email list and use your email service provider.