version: The version of the MTA-STS policy (currently "STSv1"). mode: The mode of the policy. It can be "enforce" (to enforce the policy), "testing" (to test the policy), or "none" (to disable the policy). mx: Specifies the mail servers for your domain. max_age: The duration (in seconds) that the policy is valid. A common value is 86400 seconds (1 day).