The Send Message feature can only be sent to team members for testing.
You only have a limited permission.
When the application has a formal message API permission, it can send a message to a user who is not a team member of the team.
And, only users who fall under the following can send and receive messages.
- Friend must not be hidden nor blocked.
- Friend’s Kakao Account must be linked to your service app.
- Friend must consent to the ‘Friends List in Kakao Service(Including profile image, nickname, and favorites)’ scope when linking to your service app.
- Friend must set the profile to public by selecting the ‘Show my profile on Friends list’ option. Refer to Profile visibility settings.
Please refer to the following document to obtain formal permission
Prerequisites | Kakao Developers Docs - Request permission