Comcast Corp, the largest U.S. cable operator, said on Thursday it will cap customers’ Internet usage starting October 1, in a bid to ensure the best service for the vast majority of its subscribers.

Comcast said it was setting a monthly data usage threshold of 250 gigabytes per account for all residential high-speed Internet customers, or the equivalent of 50 million e-mails or 124 standard-definition movies.

“If a customer exceeds more than 250 GB and is one of the heaviest data users who consume the most data on our high-speed Internet service, he or she may receive a call from Comcast’s Customer Security Assurance (CSA) group to notify them of excessive use,” according to the company’s updated Frequently Asked Questions on Excessive Use.

Customers who top 250 GB in a month twice in a six-month timeframe could have service terminated for a year.