A SELF-styled Muslim cleric has lost a High Court case, arguing he had the right to send offensive letters to the parents of soldiers killed in Afghanistan.

Man Monis, also known as Sheik Haron, and Amirah Droudis were charged with 13 counts of using the postal service to offend relatives of fallen diggers and a trade official killed by a terrorist bomb in Jakarta.

The letters, which were sent between 2007 and 2009, started with condolences, but the NSW Appeal Court found that they referred "to the deceased soldiers in a denigrating and derogatory fashion".

The six judges were evenly divided on ruling on the appeal, so the decision reverted to the NSW Appeal Court's decision, which ruled against Mr Monis.

Justice Dyson Heydon said many people would regard the letters as "sadistic, wantonly cruel and deeply wounding".

The case has become the centre of a free-speech debate and constitutional protection of political opinions. A counter view is that Australians should be able to be spared offensive material in the post.

In April, 2011, the men were charged under a law which "prohibits the use of a postal or similar service in a way that reasonable persons would regard as being, in all the circumstances, offensive".

These particular charges would not have been laid if the men had hand-delivered the letters.

The maximum penalty is jail for two years.