Hey Jay! To give you a solid answer on how this was possible we'd need to look at the whole session in the SMTP/Delivery and Spam Checks logging unfortunately. But, it looks like someone at Gmail passed you a spoofed EML, which is a rising complaint on the Gmail/Google email server network. These types of attempts are particularly crafty so they can take some investigation to find a pattern. Can you guys submit a ticket on this one please?
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com