This study describes the use of a true (not quasi or "natural") field experiment to test the impact of a third-party endorsement message embedded in an email advertisement on customer behavior - the message embed calls attention to reviews of third-persons' hotel-stays, mostly positive assessments, and a high overall rating (i.e. four stars in TripAdvisor for a national (Spanish) hotel chain. Applying a between-subjects experiment design, each of two versions of the email advertisement was sent to one of two random samples of treatment (embed) and control (no embed) groups of hotel's customers (n = 97,451 and 97,662, respectively). The study includes five dependent variables (DVs): clicks-to-open email; clicks-to-open offer; clicks-to-purchase room rental; number-of-nights stayed; and total revenue generated for the treatment versus control groups. Behavior was higher for all dependent measures per participants in the treatment versus control groups. Revenue per study participant was 0.083 euro for members in the control group versus 0.134 euro for members in the treatment group. Total revenue was 13,097 euro in the treatment group and 8,143 euro from guests in the control group. The share increases in the DVs may be useful as baseline share changes in future studies on behavior influences of social media embeds in internet advertisements.