This one can drive you bonkers! You’ve set the Deliverability to “All email” and you’ve checked individual user chatter settings but STILL emails are not being sent from Salesforce. WHAT IS GOING ON!?!

Odds are you need to designate a default Sender a requirement in Winter 24, “Chatter emails are no longer sent from orgs where the From Name and Email Address fields in Chatter Email Settings are blank.”

To get Chatter up and running, navigate to Setup | Feature Settings | Chatter | Email Settings and populate the “From Name” and “Email Address” fields. Ideally a “noreply” email would be used as long as someone in the company has access to click a link in an email sent to that address.