If you are using products such as Cofense PhishMe , FriendlyPhishing or Knowbe4 as Phish testing software in your organisation then these test’s will get blocked in Mimecast. These organisations will give you a list of IP Addresses e.g for Cofense PhishMe ( 52.1.96.230, 52.5.119.169, 52.20.155.14 and 52.20.128.29 ) which you will need to whitelist through Mimecast 

Login to the portal and choose Policies then Permitted Senders

Create a new policy with the below options :