There are 2 system settings that need to be verified:
1. “Allow messages with unresolved e-mail recipients to be sent”
Please note that the changes in this setting for CRM Online are not activated immediately; hence please give 10-12 hours at least for CRM to allow sending emails to un-resolved recipients.
2. If “Process emails only for approved users” (System settings > Emails) is checked then it is required to approve the primary email address of the user who is configured as the “Outgoing email owner” setting of the report scheduler.