Salesforce has made a recent change to the Chatter Email notification settings. Previously Salesforce would send Chatter email notifications from a email@example.com email address if none was specified in your Org's Email Settings.
A recent Salesforce package update has changed that functionality. Chatter Email notifications will not be sent if the following Settings are not specified.
Navigate to Setup
In the QuickFind search and navigate to Chatter
Then click on the Email Settings menu
In the Sender section, complete the following fields:
- From Name
- Email Address
Keep in mind you may want to use your Organization Wide Email Address for this Email setting as well.