How to assign proper email addresses for replys to customer

How to assign proper email addresses for replys to customer

When a user sends an estimate or invoice and the customers replies with a question, How do we direct the email to the sender and not to the master email on file?

For example: We have only 2 users.

User 1) sends an estimate. The customer relies with a question, but User 1 does not get the reply because it is sent to the Organization's Email.

Bobby