Review the customer questionnaire.
Establish trust by adding inbound and outbound emails to your allow list.
Outbound: Custom forwarding email (provided by Expel): <companyidentifier>@expel-phishing.io
Inbound: Acknowledgement and outcome response emails from Expel: email@example.com
If the above button can't be deployed, you can forward your phishing inbox submissions to Expel with original phishing email as an EML attachment to the custom forwarding email destination provided by Expel.
Send a test email submission to Expel using your customer forwarding email provided by Expel. Indicate in that email “This is a test for the Expel for Phishing service.”