Hess was arrested in Britain in 1941 after parachuting in on a peace mission
German neo-Nazis have used a personalised stamp service to send letters bearing the image of Hitler's deputy Rudolf Hess.
Deutsche Post spokesman Dirk Klasen confirmed that an order of 20 55-cent stamps had been printed by the service.
Mr Klasen said Deutsche Post would review its procedures, but that it was impossible to guarantee that unsuitable images would not slip through the net.
Rudolf Hess died in 1987, aged 93, after more than four decades in prison.
Deutsche Post's personalised stamp service was launched in February and allows customers to upload their own photos over the internet to create an individual stamp design, ordering any amount from 20 to 10,000.
The service has proved popular with people celebrating weddings, birthdays or anniversaries.
Mr Klasen said Deutsche Post does have control mechanisms in place to ensure that criminal or pornographic images are not printed, "which in most cases runs problem-free, but with the Hess portrait something obviously went wrong", he said.
Mr Klasen added that the criteria for making personalised stamps would become much stricter, but that it would be impossible to guarantee "100% security".
Hess was found dead in the grounds of Spandau prison in Berlin on 17 August 1987, with a ligature around his neck. His death was officially recorded as suicide.
He was captured in 1941 after flying into Scotland in an apparent personal bid to broker peace with Britain.
He is considered a hero by the German far-right, and neo-Nazis often use the anniversary of his death to stage demonstrations.