I've noticed there is an error logged in debug.log (when debugging is on) after a new appointment has been created by a logged in user, with a valid email address filled out in the appointment form:
[17-Oct-2017 17:42:15 UTC] WP_Error Object ( [errors] => Array ( [wp_mail_failed] => Array (  => You must provide at least one recipient email address. ) ) [error_data] => Array ( [wp_mail_failed] => Array ( [to] => Array (  => ) [subject] => [message] => [headers] => Array ( [MIME-Version] => 1.0 ) [attachments] => Array ( ) [phpmailer_exception_code] => 2 ) ) )
Seems like this is about an email that should be sent to the visitor/user, not the service provider...
Is this a bug or something missing in my settings somewhere?