Quick Tip: At least one recipient could not be resolved to a record in the system

If you trying to send an email to an address not associated to a CRM record, you received an error telling you that “At least one recipient could not be resolved to a record in the system”.

To enable sending emails to the addresses not associate to any of the CRM records, all you have do is  to update the CRM Email setting. Just navigate to Settings | Administration | System Settings, click on the Email tab, change “Allow messages with unresolved e-mail recipients to be sent” radio option from No to Yes, then click the OK button to save the system settings.

Try to send the email again and to see if it works!