If emails to Yahoo! mail or Gmail accounts are bouncing back with the subject "Mail delivery failed: returning message to sender", but you are able to send direct emails from your store's email address to those email addresses, check if the bounce back message you are receiving looks something like this:
Received: from 127.0.0.1 (EHLO host.wallabyhosting.net) (67.227.144.76)
by mtaXXXX.mail.XXX.yahoo.com with SMTPS; Fri, 01 Jan 2016 11:55:06 +0000
Received: from mailnull by host.wallabyhosting.net with local (Exim 4.86)
id 1amL4z-003XXXX-KS
for XYZ@yahoo.com.au; Fri, 01 Jan 2016 22:55:05 +1100
X-Failed-Recipients: XYZ@yahoo.com.au
Auto-Submitted: auto-replied
From: Mail Delivery System <Mailer-Daemon@host.wallabyhosting.net>
To: XYZ@yahoo.com.au
Content-Type: multipart/report; report-type=delivery-status; boundary=1459601705-eximdsn-1152973854
MIME-Version: 1.0
Subject: Mail delivery failed: returning message to sender
Email headers for Ozcart websites are set to use email authentication, so a bounce back due to false authentication is not caused due to the set up of our web servers - it is most likely caused when you are trying to send FROM a Yahoo! YMail! or Gmail account.
You need to set up your "order department" email address to be an email address at the domain name for your website. So if your website is XYZ123456.com.au then your order department email address should be something like orders@XYZ123456.com.au or sales@XYZ123456.com.au - your cart can't masquerade as coming from a Yahoo! email address, or yahoo's sender authenticity checking will fail. Only Yahoo! servers can send email on behalf of a Yahoo! email address.
