I'm using Outlook 2016 to manage email for a domain that I own. The domain is a catch-all so all email for the domain is delivered into the same mailbox.

I need the ability to change the From: address for outgoing emails. My main account is primary@example.com which is where all email for the domain is delivered. Let's say I want to send an email from sales@example.com, so I'll modify the From: address to sales@example.com for that one email. When the email is received by the 3rd party, the display name shows sales@example.com as expected. The problem, however, is that if you look in the mail headers you see this:

Return-Path: <primary@example.com>
From: <sales@example.com>
Sender: <primary@example.com>

Why does it do that? I need the email address of my primary account to be hidden from everyone I email for security reasons. If I send an email as sales@example.com, it should only show that address in the mail headers.