The default behavior is for Windows to mark file attachments with their zone information.

If this need to be corrected, configure the policy value for
User Configuration
|_ Administrative Templates
|_ Windows Components
|_ Attachment Manager
|_ Do not preserve zone information in file attachments to "Not Configured" or "Disabled".

STIG: Server
2022: https://system32.eventsentry.com/stig/viewer/V-254490
2019: https://system32.eventsentry.com/stig/viewer/V-205924
2016: https://system32.eventsentry.com/stig/viewer/V-225069

Desktop
W11: https://system32.eventsentry.com/stig/viewer/V-253478
W10: https://system32.eventsentry.com/stig/viewer/V-220955

NIST 800-53 : CM-6, SI-3, SI-4
NIST 800-171: 3.4.6, 3.14.2
CMMC v2.0 L2: CM.L2-3.4.6, SI.L2-3.14.2
PCI-DSS v4.0: 2.2.1
HIPAA SR :
HIPAA HICP : Practice 2 (Endpoint Protection)