The Chatter email setting Enable Email Reply to Chatter Notifications needs to be enabled for replies to Chatter email notifications to be posted to the organization, but this setting may not be visible in the organization setup.
Confirm whether the Allow Email Replies checkbox is missing in Setup under Chatter Email Settings. If it is missing:
If the Allow Email Replies checkbox is missing after verifying Chatter is enabled and the From address is verified, contact Salesforce Support to request that the internal setting Enable Email Reply to Chatter Notifications be enabled in the organization.
005387417

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