Yahoo is preventing some of its web email users from accessing their messages if they have an ad blocker installed. Yahoo is blocking ad blockers as part of an A/B test rather than preventing ad blockers for all web mail users. Some Yahoo Mail users are less than impressed with the move, with one calling it a ‘good start’ Other websites are beginning to detect the existence of ad blockers and request (politely) that users disable them in order to help support the site, but Yahoos approach appears more heavy-handed.”]
Source: https://grahamcluley.com/yahoo-blocking-email-access-block-adverts/