In order to allow constant contact to send emails on behalf of your domain, you need to setup DKIM authentication protocol for it. DKIM helps you verify the legitimacy of your email messages and their respective sending sources. In order to do that, you need to send an email to constant contact to inform them that you would like to authenticate your custom domain. 

Enabling DKIM signing for your custom domain can be done from the Constant Contact admin console in the Constant Contact platform.

To enable DKIM signing from the Constant Contact Admin Console, please follow these steps:

1. Click the name in the upper-right and select My Account.

2. In the My Profile section, click Campaign Email Authentication Settings.

3. Select Self-Authentication using your own domain.

4. Type in your domain name. Make sure there are no empty spaces or extra characters before or after the domain.

5. Click Generate DKIM Key.

6. Publish the Constant Contact generated DKIM key in the Valimail Enforce platform.

Go to your domain's Configuration page in Valimail Enforce and publish the newly created DKIM key.

a. Click on Add a DKIM key and fill in the info from the newly created Constant Contact key and then click Add.

7. Once you’ve completed these steps, you can begin sending authenticated emails using Constant Contact.