Microsoft is making its case against a huge fine levied against the software giant earlier this year.
The two sides are back in court to discuss the €899m the company was fined in February for violating a 2004 antitrust settlement.
The European Commission had charged that Microsoft's plan to license its code under the agreement was too expensive for most developers, making the code inaccessible and placing the company in violation of the antitrust agreement.
The company said in a filing obtained by Reuters that it felt the EU's fine was unreasonable and excessive.
"The Commission failed to take due account of the fact that the contested decision only concludes that the royalties were allegedly established by Microsoft under one particular licence," the company was quoted as saying.
Microsoft also argued that its own evidence and arguments had not been properly heard before the Commission made its decision to issue the first non-compliance penalty in its 50-year history.
The Commission did not issue an official statement on the matter, but a spokesperson was quoted by Reuters as saying that it remained confident in its decision to fine Microsoft.
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