Every email server that receives email looks at a number of things in the message to identify the message and also to decide how to manage the message.

There are 3 email addresses that matter lot.

From: "New Zealand Post Limited " <survey@nzpost.co.nz>

Reply-To: <survey@nzpost.co.nz>

Return-Path: <survey-bqqi@smzfdi.smprox.com>  

From Address

This address is used for Authentication SPF and Domain keys. It quite literally tells the recipient "Who Sent me this email" The receiving email server looks at the domain portion then performs some checks to determine if the email is coming from the correct server and is legitimate.

Reply address

This is important as it tells anyone replying to the email where to send the reply. This address does not have to be the same as the From or the Return path address but it does have to be valid. Its also very important from a consistency and branding perspective. If this address is radically different or unknown to the recipient it will generate complaints. This is why as a default for all IP isolated accounts we automatically relay these through Smartmail.

In the case of a financial institution being a man in the middle is not allowed so these emails need to go direct to the client. This is actually ideal in these cases as the sub-domain is a child of the parent domain which is trusted.

Passing checks with a sender such as a bank is only the first consideration. The appearance of emails is a major issue as the appearance of Phishing can cause as much or more damage to the senders reputation as an actual attack.

Return Path

Return Path(In the header)or Envelope sender(Our server communicates this address when sending the email without having to read the message ie. The Headers)

Can be used for SPF authentication but the main purpose is bounce handling. If this address is incorrect or incorrectly configured it will result in increase bounces or loss of bounces and damaged reputation.