This depends on each user's notification preference and on what they follow. Changing these does not affect emails sent to customers from the chatter.

• Open each user's preferences (My Preferences from the avatar menu, or Settings > Users) and set Notification to Handle in Odoo instead of Handle by Emails.
• Install the Odoo mobile app or enable browser notifications so users still get alerted in real time.
• Review followers on records: use the follow button and its subtypes to unfollow documents or receive only important events.
• In noisy channels, users can mute the channel or limit notifications to mentions only from the channel settings.
• When writing in the chatter, use Log note for internal comments and Send message only when the customer should receive an email.

After the change, internal traffic stays in Discuss while customers continue to receive emails sent with Send message.