The communications regulator has launched legal proceedings against An Post over the quality of its service.
ComReg has set a number of targets for the delivery of mail for An Post, including the delivery of 94% of standard mail within one working day.
It says An Post has fallen well short of this target, with an 84% rate in the first nine months of 2011.
However, An Post this afternoon said it was "puzzled" by the regulator's decision, saying there had been a steady improvement in its figures over the last five years.
The company also claimed its own figures did not tally with ComReg's, adding that it had "repeatedly sought clarification and increased transparency" from the regulator about how its performance was measured.