Four letters for various Newport addresses arrived at the Argus on Wednesday.
One was addressed to the chemical pathology department of the Royal Gwent Hospital, and contained a sample from a Newport patient to test for diabetes.
A hospital spokesman said the tests needed to be at the department for analysis "as soon as possible."
Another letter, addressed to the city council's finance department, ended up in our post bag despite being a recorded delivery requiring a signature.
A third was for the Crown Prosecution Service, the fourth for the Patent Office.
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A Royal Mail spokesman thanked the Argus for bringing the issue to their attention, and said the letters were re-delivered.
The spokesman blamed human error for the mistake and said staff at the Newport West sorting office are monitoring the situation.
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