This problem started recently. We're using Mailgun.org to send contact emails from our website. Non of the emails are delivered anymore. The email  postmaster@mg.doendoe.com is whitelisted is the spam settings. Here is the log:

 "delivery-status": {
        "tls": true,
        "mx-host": "mx.zoho.com",
        "attempt-no": 1,
        "description": "",
        "session-seconds": 0.5402588844299316,
        "code": 541,
        "message": "5.4.1 Mail rejected by destination domain",
        "certificate-verified": true
    },
    "recipient-domain": "doendoe.com",
    "id": "p3V2zyChTZaix4CZRwedTA",
    "campaigns": [],
    "reason": "generic",
    "user-variables": {},
    "flags": {
        "is-routed": false,
        "is-authenticated": true,
        "is-system-test": false,
        "is-test-mode": false
    },
    "log-level": "error",
    "timestamp": 1509839136.266123,
    "envelope": {
        "transport": "smtp",
        "sender": "postmaster@mg.doendoe.com",
        "sending-ip": "198.61.254.11",
        "targets": "info@doendoe.com"
    }