Why the mailbox_id field is NULL for the incidents created via email?
Oracle Service Cloud, All Versions
The mailbox_id field within the Incidents table is NULL for the incidents created via email.
The intended behavior of the application is not to populate the mailbox_id field. The default mailbox is specifically responsible for sending responses from incidents with no assigned mailbox_id, therefore, the NULL value doesn't pose any problems from a response point of view. So, in this case, the NULL value allows the application to find a legitimate outgoing mailbox from which to send the message.
Once the Outgoing Email option is enabled, the mailbox_id field will be populated. In this case, the logic is to detect the sending mailbox in order to send the response.
In conclusion, if the mailbox_id is NULL (Incoming Emails enabled), the response will be sent from the default mailbox and if the mailbox_id column is populated, the response will be sent from the associated mailbox email address (Outgoing Emails enabled).