Reference:
https://docs.microsoft.com/en-us/office365/securitycompliance/go-to-the-securitycompliance-center
https://docs.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/disclaimers-signatures-footers-or-headers
https://www.itprotoday.com/email-and-calendaring/configure-email-disclaimer-exchange-server-2016
https://docs.microsoft.com/en-us/office/troubleshoot/switch-channel-for-office-365