Internal investigations were launched after posties on rounds across the region triggered bosses' suspicions.
Chiefs said they could not reveal the cost of keeping an eye on their employees but methods included the use of CCTV cameras, test packages and tracking devices left in letters.
Daniel Tulp, information rights officer, said: "Royal Mail constantly reviews security to ensure the continuing integrity of the mail operation and we have a zero-tolerance approach to any dishonesty.
"The overwhelming majority of all letters posted arrive safely at the correct destination. It remains the case that the huge majority of our people are scrupulously honest."
In the past financial year, three postman in the region were investigated for fraud, 21 for theft, seven for postal service offences and tip-offs to bosses were received on a further 11 staff – a total of 42.
That compares with 49 probes for employees in the year before, including two fraud allegations, 22 workers accused of theft, 13 investigations into postal services offences and another 12 tip-offs – a total of 49. The full scale of investigations was revealed after a request under the Freedom of Information Act.
The probes included Peter Webb, who plundered jewellery, mobile phones, DVDs and greetings cards from mail meant for residents in Lichfield over four years. The 55-year-old was snared when Post Office investigators planted test packages on his round. He was jailed for eight months in August last year.
Bilston postman Benjamine Bryan was meanwhile convicted of hiding more than 9,000 items of mail and opened packages containing electrical items. Bryan, aged 23, of Addenbrooke Street, Darlaston, was handed a six-month prison sentence suspended for 18 months.
Dudley postman David Spiers did not even take some of his undelivered post home, with thousands of letters dumped near Dudley Zoo. The 31-year-old, of Park Moor Gardens, Dudley, was given a six-month sentence, suspended for two years.