Messenger error 10: This person cannot receive messages

Code 10 with subcode 2018108 is the third member of Messenger's unreachable-recipient family, alongside 551/1545041 and 200/1545041. Meta's table gives it the doubled-up string "This Person Cannot Receive Messages: This person isn't receiving messages from you right now." The person can be seen but not reached, and the platform will not say why.

In field experience, 2018108 skews toward the softer refusals: the person muted or ignored the conversation, moved your thread to spam, or has message-request filtering that your Page no longer passes, whereas 551 skews toward outright blocks and dead accounts. Meta documents no such distinction, so treat the difference as anecdote and the handling as identical: a permanent opt-out signal for this PSID until the person themselves re-engages.

What is NOT wrong deserves emphasis because code 10 misleads here: this is not a permission problem (bare code 10), not the messaging window (2018278/2534022), and not a bad ID (100/2018001). Your app, token, timing and data are all fine. A subcode-first error handler is what keeps these five diagnoses from blurring into one "code 10 something" bucket, and it is the difference between silently suppressing one contact and paging an engineer.

{
  "error": {
    "message": "(#10) This Person Cannot Receive Messages: This person isn't receiving messages from you right now.",
    "type": "OAuthException",
    "code": 10,
    "error_subcode": 2018108,
    "fbtrace_id": "BLBz/WZt8dN"
  }
}

The prevention is editorial as much as technical: send less, send better, and stop when engagement stops. Encode a disengagement rule, no opens or replies in N days means no more automated sends, so mutes never accumulate. Handle the whole unreachable family in one code path keyed on subcode, as laid out in the Messenger troubleshooting guide; frequency norms live in the limits reference.