A computer implemented method of detecting malicious electronic mail comprising: receiving an electronic mail message including an indication of a purported sender network domain and a Simple Mail Transfer Protocol identifier (SMTP ID); processing the SMTP ID with a classifier, wherein the classifier is implemented using a supervised machine learning method trained to classify the SMTP ID as originating from the purported sender domain based on a training data set including authentic electronic mail messages from the domain; and responsive to a classification, by the classifier, of the received message indicating that the received message originates from a sender other than the purported sender domain, identifying the received message as malicious.