The incident occurred last Thursday and Craig Smirthwaite, 36, of Cropton Road, Royston, later shared CCTV footage on social media where it was met with some outrage.
Craig said it was lucky the letter - which the worker throws in what looks like an attempt to reach the front door, after they fail to get through the gate - wasn’t an important one.
“It’s a good thing there wasn’t any personal information in it,” he said.
“They could’ve just posted it another day.
“We checked the cameras and couldn’t believe it - my five-year-old can open that gate.”
“The letter had blown to the garage and was wet through.”
A Royal Mail spokesperson said: “Every single item of mail is important to us.
“Our postmen and women work extremely hard to deliver mail to over 31 million addresses across the UK, six days a week, in all weathers.
“The vast majority of items are securely delivered and all of our employees are regularly reminded of the high standards expected of them.
“We would like to apologise to the customer concerned.
“We are looking into this incident and will be in touch directly with the customer who has made the complaint.
“Anyone who has concerns over the delivery of their mail should contact the Royal Mail customer service team on 03457 740 740 or via the Royal Mail website, www.royalmail.com.”