To route outbound email via Proofpoint, it is strongly recommended that you create or update your SPF record to include Proofpoint. The SPF record is created in your DNS (Domain Name Service).
If you already have an SPF record
Add the following entry to your existing SPF record;
If you do not have an SPF record
Create the following entry in your DNS;
Stage 4 - Routing email via Proofpoint.