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On 5/4/15, I updated my server to Version 13.4.5598. Prior to that, I think I was on Version 13.2.5511 (2015-02-02) -- (If there is a way to determine this for sure, I will, but I do not know how.)
Since 5/4/15, I have one user using Outlook 2003 that no longer receives emails via IMAP that originate from my asterisk (VOIP) server and are sent to the email account using "plus" addressing. The folder is under the root directory and not under Inbox. These emails show up in Windows Live Mail 2009, Outlook 2013 and Web Mail. Other emails have been coming through as far as I know.
I have since installed the release Version 13.4.5603 (2015-05-07). This did not fix the problem.