To authorize SAP SuccessFactors to send emails on your behalf please reach out to SAP Cloud Support team (under component LOD-SF-PLT-SEC) with the following information provided:
Your mail domain details;
(Provide a full list of the email domains used by users - there may be more than one).
SuccessFactors can also forward outgoing emails through a customer's own SMTP server(s). They only need:
the condition (recipient domain);
the customer's SMTP server's IP and port;
SMTP auth user and password if needed.
Don't forget to create a free OnDMARC account to test your configuration!