http://www.ietf.org/rfc/rfc1047.txt
Over the past few years, the staff of the CSNET Coordination and Information Center (CIC) has often been asked to help determine why a single mail message is being delivered multiple times to its recipients. In the process of tracing the problems of multiple delivery, we have discovered that many duplicate messages are the result of a synchronization problem in SMTP. There is a point in the process of delivering a message where the receiving mailer knows it has accepted the message but the sending mailer is still not sure the message has been reliably delivered. If the SMTP conversation is broken at this point, the sending mailer will be forced to re-deliver the message, even though the message has already been received and delivered by the receiving mailer.