The article deals with the problem of determining market boundaries for antitrust law enforcement in the field of telecommunications. An empirical approach has been proposed for determining the product boundaries of the market in the area of mass distribution of messages, taking into account the comparative characteristics of the types and methods of notification (informing) of end users; the possibilities of switching from one way of informing to another, including the evolution of such opportunities under the influence of technological changes; switching between different notification methods. Based on the use of surveys of customers of sending SMS messages, it is shown that the product boundaries should include not only sending messages via SMS, but also e-mail, instant messengers, Push notifications and voice information. The paper illustrates the possibilities of applying the method of critical loss analysis to determining the boundaries of markets based on a mixture of surveys and economic modeling.