Deal with Mailgun sandbox authorized recipients

[devonzuegel]

• [x] https://mailgun.com/app/testing/recipients
• [x] move from sandbox to isostamp.org
• [x] after propagation / verification of records, test using John's account

[devonzuegel]

Mailgun: "Once you've added the appropriate DNS records for verification, it can take 24-48 hours for them to propagate."