BT has been fined £77,000 by the Information Commissioner's Office, after it sent nearly five million nuisance emails to customers.
The investigation found that the telecoms company did not have customers' consent for such direct marketing.
The 4.9 million emails, sent between December 2015 and November 2016, promoted three charities.
BT said it was "disappointed" by the ICO's decision to impose a fine.
"This relates to emails concerning charitable fundraising that were sent to some of our customers in 2015-16," it said in a statement.
"There was no financial benefit to BT and minimal impact on customers - in fact, almost five million emails elicited just one complaint
"We are pleased that the ICO has acknowledged that this was not a deliberate contravention of regulations.
"In turn, we have accepted the facts set out by the ICO and have apologised.
"We immediately tightened our procedures when the complaint was originally raised in February 2017 - as part of our robust and ongoing commitment to the highest standards of data management."
ICO head of enforcement Steve Eckersley said: "Organisations have a responsibility to ensure they are acting within the law.
"Where they do not, the ICO can and will take action."
He said the inquiry had been prompted by a concerned member of the public.
"We investigated the matter and uncovered the full extent of this activity, which shows how important it is for people to report nuisance emails," Mr Eckersley said.
The emails promoted three charities:
- BT's My Donate platform
- Giving Tuesday
- Stand up to Cancer