Server response: 541 541 5.4.1 Mail rejected by destination domain

Server response: 541 541 5.4.1 Mail rejected by destination domain

I'm using mailgun to send transactional emails for our app. Sending to emails from zoho fails:

{
  "severity": "permanent",
  "tags": [
    "after_install"
  ],
  "storage": {
    "url": "https://sw.api.mailgun.net/v3/domains/mg.tabarnapp.com/messages/eyJwIjpmYWxzZSwiayI6IjVkMGIyMmFjLWFmOGItNDRlYy1hMDk1LThlZmZlNzdjYjE4MiIsInMiOiJiNDRhNTE5Y2Q2IiwiYyI6InRhbmtiIn0=",
    "key": "eyJwIjpmYWxzZSwiayI6IjVkMGIyMmFjLWFmOGItNDRlYy1hMDk1LThlZmZlNzdjYjE4MiIsInMiOiJiNDRhNTE5Y2Q2IiwiYyI6InRhbmtiIn0="
  },
  "delivery-status": {
    "tls": true,
    "mx-host": "mx.zoho.com",
    "attempt-no": 1,
    "description": "",
    "session-seconds": 0.12509417533874512,
    "code": 541,
    "message": "541 5.4.1 Mail rejected by destination domain",
    "certificate-verified": true
  },
  "recipient-domain": "tabarnapp.com",
  "event": "failed",
  "campaigns": [],
  "reason": "generic",
  "user-variables": {},
  "flags": {
    "is-routed": false,
    "is-authenticated": true,
    "is-system-test": false,
    "is-test-mode": false
  },
  "log-level": "error",
  "timestamp": 1491313348.630262,
  "envelope": {
    "transport": "smtp",
    "sender": "postmaster@mg.tabarnapp.com",
    "sending-ip": "209.61.151.224",
    "targets": "support@tabarnapp.com"
  },
  "message": {
    "headers": {
      "to": "support@tabarnapp.com",
      "message-id": "20170404134221.3582.22961.D5D915F4@mg.tabarnapp.com",
      "from": "Derek from Tabarnapp <postmaster@mg.tabarnapp.com>",
      "subject": "Thanks for installing our app!"
    },
    "attachments": [],
    "size": 7463
  },
  "recipient": "support@tabarnapp.com",
  "id": "yR0gErPKRtC-fL49ny6d_A"
}