Same behavior here with 10.12.2 ran in French.
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tell application "Mail"
activate
set selectedMessageList to selection
--> {message id 200734 of mailbox "forum AppleScript US", message id 200816 of mailbox "forum AppleScript US", message id 200676 of mailbox "forum AppleScript US"}
if selectedMessageList ≠ {} then
set selectedMessage to item 1 of selectedMessageList
tell selectedMessage
repeat with anHeader in all headers
try
content of anHeader
--> error number -1700 from item 1 of all headers of message id 200734 of mailbox "forum AppleScript US" to specifier
end try
try
name of anHeader
--> error number -1700 from item 1 of all headers of message id 200734 of mailbox "forum AppleScript US" to specifier
end try
end repeat
end tell
end if
end tell
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Yvan KOENIG running Sierra 10.12.2 in French (VALLAURIS, France) jeudi 29 décembre 2016 20:40:03