In order to enable Greenhouse as a sender in Valimail you can add it to your configuration for your org domain (top level domain) by clicking the Enable Sender button in the UI. You will also need to set up DKIM keys in Greenhouse and set up those keys in the Valimail platform at the bottom of the configuration page.
Subdomain usage: You can also set up Greenhouse to use a dedicated sending subdomain. This tends to be the preferred setup for Greenhouse. and they will typically provide you with a subdomain named gh-mail.yourcompany.com with particular setup instructions.
See this link for more information: https://support.greenhouse.io/hc/en-us/articles/201111684-Email-Verification
Note: Some customers who use Greenhouse on the top level, or org domain only, have reported that they have seen Greenhouse messages stop being sent in an aligned manner. This can happen because Greenhouse has an automated script to check for a particular include value being set in your SPF record. They cannot parse the standard SPF record we provide our customers and one of two things needs to happen:
1. you can explicitly add their include before our standard record until the validation step has occurred
2. you can contact Greenhouse Support and they will override this check.
If you have any questions about the setup and configuration please file a ticket with us and we can provide additional information.