Paul Bennett, 45, made "sexual advances" towards the iconic red object and rubbed himself up and down against it while repeatedly shouting "wow".
A shocked female eyewitness told courts the man performed the lewd act with his hands raised in what looked like a "star" position.
Bennett pleaded guilty to charges of indecent exposure and using threatening and abusive words with abusive behaviour at Wigan and Leigh Magistrates Court.
Prosecutor Katie Beattie said the drunken man's attempts to have sex with the postbox was witnessed on September 9 last year.
Paul Bennett rubbed himself up and down against the postbox and shouted "wow"
The victim was disgusted and alarmed by his behaviour. She rang the police
She said: "The victim was disgusted and alarmed by his behaviour. She rang the police."
The female complainant heard Bennett outside her window shouting and swearing with another woman during an argument.
After she left he lay down on a bench in the Scholes Precinct shopping area in Wigan before he sat up and exposed himself by pulling his trousers down.
Miss Beattie said Bennett started to make "sexual advances" towards the postbox as he walked over to it and performed the lewd sex act.
He afterwards pulled up his pants and swung on a lamppost before looking at a reflection of himself in a window.
A shocked female eyewitness said Bennett made sexual advances towards the postbox
The Wigan resident was later found by police in front of another store in the area still exposing himself.
The court heard that the drunken Bennett then became agitated and clenched his teeth as he shouted and swore at officers.
The defence's Martin Jones said: "The lady watched for some time and was ashamed, disgusted and upset and my client accepts that.
"Clearly there are issues that need to be addressed."
Bennett was handed a 12-month community order with an alcohol treatment and supervision requirement.
He was also ordered to pay £50 compensation to the victim, £150 court costs and a £60 victim surcharge.
Bennett was also told to sign the Sex Offenders Register.
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