Technologies are described for authenticating a sender identity of an online message. For example, an online message having a purported sender identity can be obtained. Various features can then be extracted from the message, including stylometric features, origin location features, attached file features for any files attached to the message, and embedded URL features. The extracted features can then be compared to a sender profile for a known sender identity matching the purported sender identity, or to one or more sender profiles for recognized suspicious senders if the purported sender identity does not match a known sender identity. The sender profile for a given sender identity can include features extracted from one or more messages previously sent by the sender identity. A global risk score for the message indicating a likelihood that the purported sender identity is inauthentic can be determined based at least in part upon the comparison.