set frontViewer to (some message viewer whose index is 1)
log frontViewer --> (*message viewer id 8*)
properties of frontViewer
Properties hasn't worked for Mail-Viewer-Windows for a long time (if at all).
On the other hand addressing individual properties works on 10.9.5:
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tell application "Mail"
set frontMsgVwr to (some message viewer whose index is 1)
tell frontMsgVwr
set vCol to visible columns
set oBox to outbox
set pView to preview pane is visible
set selMbox to selected mailboxes -- Errors if Smart Mailbox is selected
end tell
end tell
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