Does Moqui have a built-in mechanism to send notification message on a topic by email to the subscribed users?
Or should I implement a cusom NotificationMessageListener?
As of the last release of Moqui (2.1.1) and in the current GitHub code this is supported. The NotificationTopic entity has a emailTemplateId where you can specify the EmailTemplate to use for notifications on that topic. There is a simple OOTB EmailTemplate for notifications with emailTemplateId "NOTIFICATION" (defined in the MoquiInstallData.xml file).
The other side of this is the per-user configuration of whether to receive email for notifications on a given topic. The is stored in the emailNotifications field of the NotificationTopicUser entity. This can be set by users in the My Account app on the Notifications screens, or in the System app on the User Account screen.