The COVID-19 crisis brought numerous challenges to work life. One of the most notable may be the acceleration of digital transformation, accompanied by an intensification of e-mail usage and related demands such as high e-mail workload. While research quickly started to examine the implications of these changes for employees, another important group of stakeholders has been overlooked: leaders. We focus on leaders during the COVID-19 crisis and examine how COVID-19-related work intensification links to leaders' e-mail overload appraisal and finally exhaustion and transformational leadership, a leader behaviour especially needed in times of crisis. In a 5-day diary study in September 2020, 84 leaders responded to daily surveys on 343 days. Results of multilevel analysis showed that perceived COVID-19-related work intensification was positively linked to worktime spent dealing with e-mail and appraised e-mail overload. E-mail overload appraisal was positively related to leaders' exhaustion, but unrelated to their transformational behaviour. Day-specific time spent dealing with e-mail, however, was negatively related to transformational leadership. E-mail overload appraisal mediated the relationship between COVID-19-related work intensification and exhaustion. Turning the focus on leaders during the COVID-19 crisis, our study has important implications for the design of work of leaders in times of crisis and beyond.