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I'm going over some SMTP log files from an open relay server (IIS 6 SMTP Virtual Server), used to collect and route all automated email within a private network, to an internet smarthost (qmail at another ISP). Using SawMill 8.5 to parse the logs, it appears that there is a big difference between the number of messages delivered from my server, and the number of emails that the internet smart host sends out. For a period where I have delivered 2000 emails to the smart host, the smart host has delivered 3000 emails to the internet. I guess this has something to do with emails having multiple recipients. So my question is this:

When does a single message with multiple recipients get split and routed to the individual recipients' MX? At submission? At the last hop before the internet? Or is this MTA specific?

Let me now if I need to clarify anything

Mathias R. Jessen
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I'd imagine the MTA will split up a message for different MXes when it has to. From my experience with Sendmail behavior, this will be when it encounters different next MTAs in the recipient list of a given message. If there's just one next MTA it talks to for a given message, there will be just one transaction.

With a smarthost configuration, the next MTA in all these cases will be the qmail at the other ISP, so there's no need to split up messages at that point.

cjc
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    +1 It depends on the software. But they all do one of three: 1. Always split messages for each recipient. 2. Split based on individual domain names of recipients. 3. Split on individual MX of each recipient. – Chris S Jan 23 '12 at 21:24