The growing ubiquity of mobile telephony able to send e-mail raises new questions, and renews old issues, about the effect of the medium on a message. This article reports experimental results testing effects of user- and medium-generated cues on perceptions of message senders. Grounded in warranting theory, we assess the interaction of low- and high-warrant cues on perceptions of the sender's professionalism, hypothesizing senders of grammatically accurate messages are perceived as more credible. However, we also hypothesize an interaction between grammatical accuracy and the system-generated high-warrant cues from the mobile device used to send the message. Responses from 111 students assessing the credibility of an e-mail sender indicate that, although a message's user-generated content (grammatically accurate vs. erred) influences receiver's perceptions, negative attributions are attenuated by cues reflecting the transmission medium (i.e., a message's mobile signature block). Findings offer theoretical implications for warranting theory. Additionally, results suggest practitioners need to craft a message and indicate the transmission medium strategically to mitigate any impacts on attributions of professionalism to message receivers.