If the meeting you are participating in is hosted by another organization, whether you can chat in the meeting depends on the policy settings of the other meeting organizer.
"External meeting chat", when this is turned on, people can read or write messages in external meeting chats from untrusted organizations. Note that this policy is intended for untrusted organizations.
If both organizations have configured a two-way trust relationship in external access, your users can speak in the meeting chat as long as the other organizer has enabled the Meeting chat function.
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