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WikiPedia: Tem_thư http://pe.usps.com/text/dmm300/608.htm#wp1101003 http://hanoi.vnn.vn/goctem/lienket/ http://www.vnpost.vn/Chuy%C3%AAntrangTemXB/tabid/1... https://books.google.com/?id=EBI4brLkz7AC&pg=PA118 https://commons.wikimedia.org/wiki/Category:Stamps... https://www.bbc.co.uk/news/business-48844278