The delivery company has been given five weeks to stop parking its commercial vehicles in a car park intended for private use at its site in Skyliner Way, Bury St Edmunds.
It comes amid growing issues with staff parking in streets around the Moreton Hall estate, where the office is based.
Leader of West Suffolk Council Cliff Waterman has confirmed a breach of condition notice has been served on Royal Mail due to the alleged use of the car park in contrary to the original permission.
“We can confirm that a breach of condition notice has been served on The Royal Mail,” said Mr Waterman.
“The notice gives Royal Mail five weeks to stop parking fleet and commercial vehicles in an area it has designated under a planning condition for use as private car parking. Any failure to comply is a criminal offence which could result in court proceedings and a fine for the business.”