The airline Flybe has been fined £70,000 for sending more than 3.3 million marketing emails to people who had opted out of receiving them.
The emails, sent in August 2016, advised people to amend out-of-date personal information and update their marketing preferences.
They also gave people the chance to enter a prize draw.
But the regulator said Flybe should have obtained people's consent before sending the emails.
"Sending emails to determine whether people want to receive marketing, without the right consent, is still marketing, and it is against the law," said Steve Eckersley, head of enforcement at the Information Commissioner's Office.
"In Flybe's case, the company deliberately contacted people who had already opted out of emails from them."
Flybe told the BBC it wanted to "sincerely apologise" to affected customers.
"We can confirm that appropriate mechanisms have already been actioned to ensure that such a situation does not happen again," it added.
Honda also fined
The ICO has also fined carmaker Honda Motor Europe £13,000 after a separate investigation found similar breaches.
The company sent 289,790 emails to clarify customers' choices for receiving marketing, but did not secure their consent.
"The firm believed the emails were not classed as marketing but instead were customer service emails to help the company comply with data protection law," the ICO said in a statement.
"Honda couldn't provide evidence that the customers had ever given consent to receive this type of email, which is a breach of privacy and electronic communication regulations."
Honda said it was disappointed with the decision and that it had acted with "the best data protection practices in mind".
It added: "It is also important to highlight that we have already taken steps to address the concerns that the ICO has raised, and we are pleased that the ICO has recognised that any breach of the PECR by Honda was not deliberate nor intentional."