Read EMail as plain text must be enforced.

Outlook can display email messages and other items in three formats: plain text, Rich Text Format (RTF), and HTML. By default, Outlook displays email messages in whatever format they were received.

Verify the policy value for User Configuration >> Administrative Templates >> Microsoft Outlook 2016 >> Outlook Options >> Preferences >> E-mail Options "Read e-mail as plain text" is set to "Enabled".

Procedure: Use the Windows Registry Editor to navigate to the following key:

HKCU\Software\Policies\Microsoft\Office\16.0\outlook\options\mail

Criteria: If the value ReadAsPlain is REG_DWORD = 1, this is not a finding.

Set the policy value for User Configuration >> Administrative Templates >> Microsoft Outlook 2016 >> Outlook Options >> Preferences >> E-mail Options "Read e-mail as plain text" to "Enabled".