http://www.rfc-editor.org/rfc/rfc1047.txt
RFC 1047: Duplicate messages and SMTP, C. Partridge1988-02-01T23:00:00-00:00An examination of a synchronization problem in the Simple Mail Transfer Protocol (SMTP) is presented. This synchronization problem can cause a message to be delivered multiple times. A method for avoiding this problem is suggested. Nodding familiarity with the SMTP specification, RFC-821, is required.