Large introductory courses are at a disadvantage in providing personalized guidance and advice for students during the semester. We introduce E2Coach (an Expert Electronic Coaching system), which allows instructors to personalize their communication with thousands of students. We describe the E2Coach system, the nature of the personalized support it provides, and the features of the students who did (and did not) opt-in to using it during the first three terms of its use in four introductory physics courses at the University of Michigan. Defining a 'better-than-expected' measure of performance, we compare outcomes for students who used E2Coach to those who did not. We found that moderate and high E2Coach usage was associated with improved performance. This performance boost was prominent among high users, who improved by 0.18 letter grades on average when compared to nonusers with similar incoming GPAs. This improvement in performance was comparable across both genders. E2Coach represents one way to use technology to personalize education at scale, contributing to the move towards individualized learning that is becoming more attainable in the 21st century.