If you want to "Send Email On Behalf Of" someone else, should you use the 'Sender' heading or the 'x-sender' heading?
Outlook 2007 seems to only pay attention to the 'Sender' heading, if it's present.
On the other hand, there are those who believe that the 'Sender' heading has been deprecated in favor of the 'x-sender' heading.
You could use both at the same time. "Sender" and "X-Sender" are not mutually exclusive and neither of them is deprecated.
According to RFC2822 - Internet Message Format X-headers ought to be nonstandard and provided for information only but in real life this convention does not hold. Using "Sender" and "X-Sender" simultaneously gives you an edge on systems that support only one or the other.