Symptoms
Unable to send email to a particular SharePoint group with error:
"The email address, Null or Empty, does not meet the requirements."
Diagnoses
Sending email to other SharePoint groups work fine. It turns out, the SP group contains a total of 15 members whereas the SmartObject (Users and Groups.Get E-mail For Group Users.E-mail) used to populate the To address on the Email Event only returns 14.
Resolution
After removing the member from the group whose email did not appear, this has been resolved