This KB is for Email Security Support Engineer to enable Full History Logging which required in few corner cases
Full History logging can help to collect email sample/file attachment before being analyzed for threats
'Save a Copy of every email' option under Manage | System Setup | Server | Advanced provides Support team with the ability to save a copy of every email that enters or leaves the organization.

NOTE: If the navigation or the screenshot looks different from the one mentioned above , you may be in an older firmware version and would require a firmware upgrade. Please refer the link below to upgrade the firmware to latest version.
https://www.sonicwall.com/en-us/support/knowledge-base/170504270079039