Execute an AppleScript in an AppleScript action:
This is the script text:
tell application "Mail"
activate
repeat (count (windows)) - 1 times
set miniaturized of window 2 to true
end repeat
end tell
Note: We have to repeatedly minimize window 2
because when the first window 2
is minimized apparently the window that formerly was window 3
becomes window 2
and so on.