Impersonation Protection in Microsoft Defender for Office 365 is part of the Anti-phishing policies, designed to take action if an external email comes in with a match, or near match, to the display name of an employee.
The actions you can take when a match is made are:
- Redirect message to other email addresses
- Move message to the recipient’s Junk Email folders
- Quarantine the message
- Deliver the message and add other addresses to the Bcc line
- Delete the message before it’s delivered
- Don’t apply any action
What I wanted to do, was deliver the message and add other addresses to the bcc line. This could be used to send a copy of the email to helpdesk for investigation, as Impersonation Protection tends to get a lot of false positives from services that like to use people’s actual names from emails they generate, or from people using a personal account to email other employees.
What I found was that the action was applied, but the email was then delivered to the Junk Email folder. If I wanted that to happen, I would have selected the ‘Move message to the recipient’s junk email folders’ option. After logging a case with Microsoft, I found out why.
Any time an email is detected as an Impersonation Protection, and the mail is still allowed to flow through, it will set the header as SCL 5. As per Office 365 standards, this will deliver the email to the recipient’s junk mail folder.
It makes the choices on what actions to take in the Impersonation Protection settings rather misleading; but there is one option that’s still reasonable – Quarantine the message. This should trigger a fairly quick quarantine digest to the recipient for review, allowing them to review and decide if it should be released. If released, it will then deliver to the Inbox rather than Junk Mail.