As of Winter '24, Chatter email notifications will only be sent if admins provide a From Name and Email Address on the Chatter Email Settings. This enforcement will increase data security and it applies to all Salesforce orgs.

To ensure that Chatter email notifications continue being sent, provide a From Name and Email Address. Navigate to Setup → Chatter → Email Settings.

There is no need to add all users' emails. This can be a general email or the administrator’s information.