SmarterMail 8.4 with Outlook 2010 using POP3, some mail not downloaded

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http://serverfault.com – We have a very strange situation. We are subscribe to SmarterMail 8.4 for hosting our email services, the email hosting panel is WebsitePanel + Plesk Panel. Our colleague is using Outlook 2010 with POP3 setting. We had noticed that some of the mail was found in the webmail, but it is not found in the Outlook client. The webmail got 50 emails showed, but outlook only showed 42 emails, it from different sender also. Thing for sure, - My colleague did not delete the email. It is just not downloaded to the outlook client, What will be the possibility reason that will happen? As my understandi (HowTos)