Customer mistreatment is a growing issue for service organizations. The present study specified two forms of customer mistreatment behaviors: aggressive and demanding mistreatment and tested their proximal and lagged effects in predicting within-person fluctuation of service employees' emotional well-being. An archival data set was used to test the hypotheses. One thousand one hundred and eighty-five daily surveys were collected from 149 customer service representatives from a call center for 8 weekdays. Multilevel analyses were conducted to test the hypotheses. First, drawing on the cognitive appraisal model of emotion, theory for mood development, and resource perspective, the present study examined both proximal and lagged effects of customer mistreatment on employees' emotional well-being (i.e., daily emotional exhaustion and negative mood in the next morning) and the role of daily emotional exhaustion in mediating the lagged association between daily customer mistreatment and employees' negative mood in the next morning. The mediation model was largely supported by the current sample. Second, positive treatment by customer was demonstrated to be a significant moderator in buffering the detrimental effect of demanding but not aggressive customer mistreatment in predicting daily emotional exhaustion. Third, employees' on-line emotion regulation strategies (i.e., surface acting, deep acting, and natural expression) and off-line emotion regulation strategies (i.e., rumination and social sharing) were tested as moderators on the negative relationship between customer mistreatment and employees' lagged negative mood. The current findings supported some of these moderation effects but were not consistent across aggressive mistreatment and demanding mistreatment. Theoretical and practical implications were discussed.