We make SMTP, or email service an optional component. The system still works even if it's not configured.
Notifications will be send through email in the following cases:
Workspace member invitation/join/reject, etc.
Doc @mention behavior
Authentications
You have to setup SMTP correctly to make everything works as expected.
Go visit Admin Panel -> Settings -> Notification
, you will see the form where you can input your SMTP host and credentials.
There is also a [Send Test Mail] button to let you quickly test you settings' correctness.