Grayed out "Redirect" option in OSX Mail.app Running recent Mavericks (10.9.5)

I am wondering why I can only sometimes redirect messages, as opposed to forwarding, as in principle being offered via "Message > Redirect".

Sometimes this works, sometimes not, i.e., "Message > Redirect" is a grayed out option. 

Ts there a somewhere documented pattern for this behaviour?
Redirect is disabled for Exchange accounts (last §), presumably because the sender address is enforced by the Exchange server (to prevent spoofing).