This error will occur when Mailgun delivers your emails using a low-rated IP address. Low-rated IP addresses may cause the ISP to send the email to spam or reject it.
First, check the sender score of your IP address on Mailgun using a tool like Sender Score. If the score is less than 70% (this is the case for most Mailgun accounts on the free plan), send a support ticket to Mailgun with a request to assign your account a sender IP with a higher reputation.
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