Oracle publishes Sender Protection Framework (SPF) policy that identifies the Oracle server IP addresses and subnets that are permitted to send cloud services emails.
You can use this information to assess the validity of the messages to determine whether or not to accept them. Additionally, you can use the information as a part of the message protection services.
Add the following in your SPF record to take advantage of this protection:
"v=spf1 include:spf_c.oracle.com -all"