A forum search for DMARC shows other discussions about email providers (including hotmail, yahoo, aol, etc) implementing tougher spam-control rules this year.
The solution is:
1. Set Email Transport method to SMTPAUTH, and provide your SMTP username+password, host, port, etc settings.
2. Ensure "Email must send from known host" is enabled
3. Ensure that your "Email Address (sent FROM)" address is a real email address on the same domain as your webstore (ie: if your store is "marys_pots.com" then use an email address like sales@marys_pots.com ) ... because trying to send from gmail.com or hotmail.com is seen as spoofing and falsifying the email origins.
4. Thus it requires than you have an actual mailbox on your domain (ie: the sales@marys_pots.com account must be real)
Additional reading about email issues:
http://www.zen-cart.com/wiki/index.p...Email_Problems
http://www.zen-cart.com/wiki/index.p...Of_Lost_Emails
... and search for DMARC, both here in the forums and elsewhere online.