Procedure for Document and Record Control

This is what the Paragraph states:
Each external document, which is necessary for the planning and operation of the ISMS, must be recorded in the incoming mail register. The incoming mail register must contain the following information: (1) document number, (2) sender, (3) document name, (4) date of receipt, (5) name of the person whom the document has been forwarded.

Answer:

An incoming mail register is a record used to identify any information received by the organization from external parties, either on physical or electronic media. Examples of incoming mail are hard copies of equipment manuals you use or standards you must comply with, or a customer e-mail requiring changes in a project's specification.

Your understanding is right about not all mail coming into the organization need to be in the incoming mail register. Examples of external documents necessary for the planning and operation of the ISMS are the ISO 27001 standard, customer requirements for service delivery, and audit reports from your certification body.