If you want to know who REALLY sent you an email and not what Outlook is reporting who sent you an email, you just need to check what Outlook actually received in its header. The header contains all kinds of useful tech bits and is often modified by systems that touch and retransmit the message (such as an external anti-spam service from your internet provider).
To see a message header in Outlook 2007 2010 or 2013:
- Open the message in question in a different window (i.e. double click on it to get it out of the preview pane)
- Click FILE, PROPERTIES
- Read the header – personally, I find the header display window to be too small so I often just copy and paste the data into notepad