Just a note on setting the sender of a mail message, for those of us with multiple email accounts. Hope some people might find it of interest.
It used to be impossible to set the sender of a new mail message in AppleScript. Instead, I used a complicated GUI script that clicked the sender field and typed the beginning of the address I wanted. (I have quite a few.)
Upgrading to 10.11 seems to have killed that, but It was pretty fragile anyway. I overrode ⌘N to invoke a set of KM macros, using the conflict palette to pick the one I wanted. The main point was to force me to choose, rather than accidentally sending from the wrong address.
Two things changed that make it possible for me to abandon the GUI script solution (though not the use of a conflict palette) and use a much simpler approach:
I can use the beautiful KM Send Mail action (which I use with And Save Open, rather than And Send, and filling only the sender field).
To my surprise, it is now possible to set the sender of a message via AppleScript:
set the sender of theMessage to "sender address"