It's hard to send letters home; to be cut off from news of one's hometown or family, implying a long separation and lack of communication.
他独自在外闯荡多年，感叹乡书难寄，与家人已久未联系。
Having ventured out alone for many years, he sighed that hometown letters were hard to send, as he had not been in touch with his family for a long time.
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