A system and method for preventing e-mail spoofing, in which a receiving e-mail checking server system sends a message to a confirmation server associated with a network domain of the sending system of a received e-mail message, to determine if the sender transmitted the message. The e-mail checking server sends a confirmation request e-mail, including a transmission time or unique message “key” associated with the received e-mail, to the sending domain's confirmation server. When a confirmation request is received at the confirmation server, it replies with an indication whether the message was sent at the time indicated in the confirmation request, and/or whether the message key matches that of a previously transmitted message. The confirmation server checks whether the message was in fact sent based on stored values corresponding to the send time and/or message key stored for the confirmation request message. A message may further include indication that its origin can be confirmed. A receiver may maintain indications of domains capable of confirming sent e-mail messages, and determine a problem has occurred if no confirmations are received from a domain having that capability. An e-mail sender may mark domains from which it expects to receive e-mail confirmation requests, and determine a problem has occurred if e-mail messages sent to such domains fail to send confirmation requests.