If you need a mail system see Appstore for pre defined mails or end-user customizable mail.
If you need messages in the application, for example a user logs in and on his/her homepage sees all required actions or messages, you can model that with a an entity message (name, date, description) that is associated with built-in account. It has a status (open, processed). If a user logs-in the homepage a datagrid is displayed with an Xpath
Thus he sees his own messages. If more privacy is required you can change access rules and add attribute 'Private'.
The access rule can be
[(Message_Account=$currentuser and Private) or (not Private)]
All screens are now secure and microflows if you tick 'Apply entity access'