-The customer contact class (or classes, as a comma-separated list) to send
-the message to. If unspecified, will be sent to any contacts that are marked
-as invoice destinations (the equivalent of specifying 'invoice').
+This field contains a comma-separated list. This list may contain:
+
+- the text "invoice" indicating contacts with invoice_dest flag should
+ be included
+- the text "message" indicating contacts with message_dest flag should
+ be included
+- numbers representing classnum id values for email contact classes.
+ If any classnum are present, emails should only be sent to contact_email
+ addresses where contact_email.classnum contains one of these classes.
+ The classnum 0 also includes where contact_email.classnum IS NULL
+
+If neither 'invoice' nor 'message' has been specified, this method will
+behave as if 'invoice' had been selected