I’ve also been struggling with this issue a lot on the Perfect Product blog.
Can’t signup with gmail.com addresses. Outputs the same glitch you’ve mentioned above:
Reaching out to Mailgun customer support hasn’t been much help either because my google domains DNS seetings and mailgun email settings are correct according to both companies documentation.
Not very fun when I can literally learn how to develop software products more easily than solve my blog’s glitch adding new subscribers.