To authorize Proofpoint Essentials to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanisms used by Proofpoint depend on your region as shown below.
For example, if you needed to authorize the US region your SPF record would look like this:
"v=spf1 a:dispatch-us.ppe-hosted.com ~all"
DKIM signing is now available in Proofpoint Essentials, please speak to their support team to enable it for your account.
Create a free OnDMARC account to test your configuration.