Thanks for reaching out. I don't know anything about the application that your clients are using, but typically how you'd do this is to set the "Reply-To" header within the email content. That way, when the user responds to the email, they will be replying to a specific email address.
Here is an example of how the EML might look with a custom "Reply-To" header:
Subject: Quote for Your Order
Content-Type: text/plain; charset="utf-8"
Here is the quote you requested...
In this example, the email is sent from firstname.lastname@example.org, but the "Reply-To" header is set to Amy@companyxyz.com. When the recipient replies to this email, the response will be directed to Amy's email address.
Please let me know if you have any other questions or need further assistance.