Teams error 403 ConversationBlockedByUser: ConversationBlockedByUser - user blocked the bot

The person on the other end blocked your bot, and the Connector now rejects every write into that conversation. The status table documents the code as ConversationBlockedByUser with the message "User blocked the conversation with the agent." (earlier doc versions say "bot") and explains: "User blocked the agent in personal chat or a channel through moderation settings." Retry is documented as No, and the developer action is specific: "Delete the conversation from cache. Stop attempting to post to conversations until interaction with agent is explicitly initiated by a user in the conversation, indicating that the agent is no longer blocked."

Teams gives users a block control on personal bot chats, and channel moderation settings can shut bots out of a channel's replies. Either way there is no webhook, no conversationUpdate, and no Graph event to tell you it happened - the 403 on your next send is the only notification you get. That makes this code (together with its proactive-messaging sibling MessageWritesBlocked) the de facto block-detection mechanism: production notification bots treat the first 403 of this kind as an unsubscribe event.

Nothing about your app registration, manifest, or admin policy is implicated. The same bot keeps working for every other user; only the blocking user's conversation is closed to you.

{
  "error": {
    "code": "ConversationBlockedByUser",
    "message": "User blocked the conversation with the agent."
  }
}

Users block bots that message too often or say too little. Make every proactive message answer three questions - why am I getting this, what changed, what can I do - and give a visible opt-out that does not require blocking. Watch your block rate as a product metric: a spike after a new notification type ships is user feedback. Frequency guidance and the message-design checklist live in Microsoft's proactive messaging best practices, summarized in proactive messaging constraints.