Incident Response now includes external users (outside your organization) as part of impact radius when an incident is created. You can view external and internal users under “USERS” tab while viewing incident details. However, we cannot track if these external users clicked on a link (if one was present), forwarded the email, opened an email or replied to the email.

https://campus.barracuda.com/product/forensics/doc/79463566/reviewing-incidents/