Dr. I Doctor
Dr. I Doctor's Informational Juggernaut
June 1, 2008
Duplicated Incoming Email Messages
Dear Doctor,
We're a total i5/OS shop and run several different kinds of mail servers: Lotus Notes, CommuniGate Pro, and the native i5/OS SMTP server. Each server provides e-mail for a specific suborganization of our enterprise. Recently, we've had a strange problem that affects all three servers: Incoming mail messages occasionally get duplicated, or worse. I've seen up to six copies of the same message. I see no pattern in where the duped messages originate, and with three completely different mail servers, it's hard to believe this is our problem.
Gentle User,
The problem you see is a common artifact of the Simple Mail Transport Protocol (SMTP) used to deliver messages to your servers. They don't call it "simple" for nothing.
SMTP is generally reliable, but it has serious problems when faced with network congestion or high server workloads that slow the interaction between sending and receiving server. When a sending server establishes an SMTP session with one of your servers and transmits an e-mail, the sender expects a "250" reply confirming reception. If your mail server is too busy to respond quickly, or severe network congestion requires one or more retransmissions of the "250" reply, the SMTP session could time out, leaving the sender with the impression that the message wasn't delivered. So it sends it again. And again.
Since multiple originating mail servers exhibit this behavior, the problem is likely with your network. Either the network itself is too congested at the Internet border, or your mail servers are all very busy. One cause for pointless busyness among related mail servers is mail looping from one server to another — a problem you can detect by monitoring the mail queues on each server.
Posted by mbeckman at June 1, 2008 4:00 PM
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