Set Up DKIM Signing For CF7 Messages

How can I DKIM-sign my CF7 autoresponder messages?

I’m using CF7 for my contact form on the site linked with this topic. I’m using the Email 2 feature to send confirmations to people who use the contact form. I want to ensure that these emails are DKIM-verified.

However, my regular email goes through Google, not through the server where the website is hosted. So the DKIM signing I’ve set up doesn’t seem to get invoked when the CF7 confirmations are sent.

How can I ensure that the DKIM signature is added to the outgoing messages from CF7?

The page I need help with: [log in to see the link]