If you've got troubles with mails, often mail-headers help to find out what the problem could be.

Each mail-client hides the mail-headers in a different location, here some links to help you find the mail-headers:

Outlook 2007 / 2010 / 2013 (english)

Outlook 2010/2013 / 2007 / 2003/2002/2000 / GMail (english)

Outlook Express / Outlook 2010 / Windows Live Hotmail/MSN / GMail / Yahoo! Mail / Apple Mail (english)

Outlook 2010 (german/deutsch)

Mozilla Thunderbird / Outlook 2010 / Outlook Web App (OWA) (german/deutsch)

Apple Mail / Outlook 2010 / Mozilla Thunderbird / Windows Live Mail (german/deutsch)