The CWU members are in a dispute with their employer Royal Mail over pay and efficiencies.
More than 100,000 postal workers have walked off the job in what has been described as the biggest strike of the summer so far.
Members of the Communication Workers Union (CWU) at Royal Mail said that the 2% pay rise given to them by management was not good enough, and they are instead seeking an amount that is "dignified" and "proper".
Some 97.6% of members voted in favour of the strike action, which will continue on Wednesday 31 August, Thursday 8 September and Friday 9 September.
Dave Ward, general secretary of the CWU, said: "There can be no doubt that postal workers are completely united in their determination to secure the dignified, proper pay rise they deserve.
"We can't keep on living in a country where bosses rake in billions in profit while their employees are forced to use food banks."
He pointed to the company's adjusted operating profit in the year ending March 2022 of £758m and its decision last November to hand shareholders £400m in dividends, saying: "Our members won't accept pleas of poverty from the company.
"Postal workers won't meekly accept their living standards being hammered by greedy business leaders who are completely out of touch with modern Britain.
"They are sick of corporate failure getting rewarded again and again."
Royal Mail said that it would prioritise the delivery of
COVID test kits and medical prescriptions and deliver as many Special Delivery and Tracked24 parcels as possible.
A spokesperson said: "We cannot cling to outdated working practices, ignoring technological advancements and pretending that
COVID has not significantly changed what the public wants from Royal Mail.
"While our competitors work seven days a week, delivering until 10pm to meet customer demand, the CWU want to work fewer hours, six days a week, starting and finishing earlier.
"Their plans to transform Royal Mail come with a £1bn price tag, are predicated on a wholly unrealistic revival in letter writing and prevent Royal Mail from growing, and remaining competitive, in a fast-moving industry.
"Our future is as a parcels business. We must adapt old ways of working designed for letters to a world increasingly dominated by parcels, and we must act fast.
"We want to protect well-paid, permanent jobs long-term and retain our place as the industry leader on pay, terms and conditions. That is in the best interests of Royal Mail and all its employees."