As I noted the other day, I was still having trouble getting one of my Macs to download all of the email from an Exchange 2010 server, so that it could go into my triple-redundant email archives.   This morning, when checking my calandar on the offending computer, I finally discovered what the problem was: the connection to my old exchange account was still listed under the Preferences->Accounts pane in iCal:

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iCal account configuration pane

Basically, there was a duplicate entry here.  Since Exchange manages calendaring through the email server, and the old account was active, it seems to have prevented the download of any email earlier than the last unanswered appointment request in my inbox.  So, to reiterate my original point:

No reasonable end-user of email can be expected to keep track of all of the dependencies and relationships between server types and capabilities, client types and capabilities, IT administrator settings, end-user settings , inherent software limitations, and user behaviors to ensure that a complete record of  messages are actually being saved according to a plan.