Not parent but here's a couple of not fully thought out ideas:

1. Have the signer upper send a provided email from their own Gmail account to their own company account. Then show the message's DKIM headers to blind. Now Matt has to find emails from employees personal mailboxes to their corporate mailboxes but he can't tell what the content is about.

2. Employ a graph of email forwarders (humans) selected from existing blind users. Use them to forward challenge email payload to the signer upper. Now Matt has to find emails from anyone to anyone and still doesn't know what the content is about.