Royal Mail has been fined £12,000 for sending 327,000 nuisance emails to people who had opted out of receiving such information.
As a result, Royal Mail broke the law, the Information Commissioner's Office (ICO) said.
The offending messages were sent on two dates last July.
Royal Mail tried to claim that the emails - which advertised lower prices for parcels - were a "service" rather than marketing materials.
However, the ICO disagreed.
"Royal Mail did not follow the law on direct marketing when it sent such a huge volume of emails, because the recipients had already clearly expressed they did not want to receive them," said the ICO's head of enforcement, Steve Eckersley.
"These rules are there for a reason - to protect people from the irritation and, on occasions, distress nuisance emails cause. I hope this sends the message that we will take action against companies who flout them."
Royal Mail apologised, and said it would be launching additional training for its staff.
"We take the privacy of our customers extremely seriously," the company said.
"We are very sorry that we let some of our customers down on this occasion. Following this incident, we have tightened up our processes and governance measures still further."