By Steve Rosenberg
BBC News, Berlin
Mr Zumwinkel was arrested last year
The former head of Germany's Deutsche Post, Klaus Zumwinkel, has gone on trial for tax evasion.
He pleaded guilty to charges of avoiding paying nearly 1m euros in taxes (more than £900,000) in 2003-07.
He is one of the most high-profile German executives to be prosecuted for tax fraud.
Prosecutors allege that Mr Zumwinkel used a foundation created in Liechtenstein to defraud the state by transferring huge amounts to it.
Mr Zumwinkel was arrested last year after his name and bank details had featured on a data disc sold by a Liechtenstein bank official to the German secret services.
On the opening day of the trial, Deutsche Post's former chief executive admitted evading taxes.
He described his actions as the biggest mistake of his life and promised to bear the consequences.
Following his arrest, the tiny principality of Liechtenstein came under intense international pressure to make its financial institutions more transparent.
This was to discourage other wealthy businessmen from around the world from using Liechtenstein as a tax haven.