To determine this, they conducted a series of experiments in which hundreds of participants were asked to read work-related emails from unknown people, then evaluate them based on both the competence and "warmth" of the person who sent it. And while the messages were all similar, those that contained smileys not only didn't have any effect on how warm a person seemed, but actually reduced the perception of the sender's level of competence.
As Dr. Ella Glikson, one of the lead researchers behind the study explained, "When the participants were asked to respond to e-mails on formal matters, their answers were more detailed and they included more content-related information when the e-mail did not include a smiley." So essentially, including a smiley in a message flags you as someone less worthy of time and energy. "We found that the perceptions of low competence if a smiley is included in turn undermined information sharing," Glikson said.