Outlook 2011 Mac (Using Hosted Exchange) missing senders email / reply address

On some received emails the senders email / reply address is missing - which means there's no address to reply to. The sender is in my address book, but on the received email frame a '?' appears next to the senders name. Clicking to open the contact  view the email address doesn't link through to the contact/sender and only the senders name is displayed. The address is only missing on the Outlook 2011 desktop application - the senders email / reply address shows correctly when viewing the email in Outlook Web access. How can I resolve this - Cheers!