SYMPTOMS

When a list message is received by Outlook clients it displays the "On Behalf Of" text on the sender line. 

CAUSE

The cause of this is due to the feature "Delegate Access" in Outlook this feature looks for the original recipient address in the header of the message and checks to see if it matches the FROM address.  When there is a difference between the sender address and the from address the from line will include;

FROM: <Original Message Recipient/Sender> On Behalf Of <From Address>

SOLUTION

When a reply or a forward is done the original recipient is written into the header this action can not be disabled.  Outlook recognizes that 'from" and "sender" are different and thinks it's being helpful with the "On behalf of" message which in an Exchange/MAPI environment it is due to the fact that the Delegate Access feature is designed for this configuration.  When using Mozilla Thunderbird you still see "From" and "Sender" in the normal brief header but it doesn't include "On behalf of" when you hit reply so this is a possible work around.