[Exim] Figuring max smtp sessions

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Does anyone have a guideline they use for determining the max_smtp_sessions for their server? I had mine set to 200 and then recently doubled it to 400 and my CPU usage dropped significantly. I'm assuming it has something to do with less load on the server as messages don't get backed up in the queue, but that's just a guess. I'm afraid it may also be due to drops in traffic after a much-needed audit and weeding of old mail accounts.

Thanks,
Mike Oliveri
Systems Administrator
UTI Systems, Inc.
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