To add Guardian Mail. Add the following code below as described in the following Microsoft Support Documentation: 
Add-IPBlockListProvider -Name "Abusix" -LookupDomain <APIKEY>.combined.mail.abusix.zone -RejectionResponse "Source IP address is listed by Guardian Mail" -AnyMatch $true
Replace “APIKEY**”** with “Your API key” from your account in app.abusix.com.