Question

______ is not visible to us, rather it’s used internally by the e-mail program to track replies​

Answer 1

"E-MAIL TRACKING" IS THE RIGHT ANSWER....

Answer 2

Answer:

Email Threading Unique Id of email

Explanation:

Every email contains a unique message-id, which is automatically included in the references of the email header. When we receive an email, we loop in the references and message-id to see if we can find any similar conversations. If we do, we'll thread the incoming message to the existing conversation we’ve just found. If no message is found, then we create a new conversation.

This unique id of email can be seen by a skilled programmer or developer only.