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It sounds like you have not verified your sending domain in Mailgun. Anymail is reporting the API error directly from your ESP, Mailgun, which is complaining they can’t find the domain you’re trying to send from.
Sign into your Mailgun account and check the domains list in their dashboard. Is mg.mailinator.com listed there with a green "Active" badge? If not, add it if necessary, and then follow Mailgun’s instructions to verify the domain.
Source:stackexchange.com