Outlook 2010 - Sending to local accounts does not work sometimes

In Outlook 2010, when I send to a local user using the name before the @ sign, the email frequently gets returned as undeliverable.  If I select the same portion of the user name from the local address list, the email is always delivered.  Why can't I send to the prefix name on all occasions?
WDravingIT ConsultantAsked:
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Raymond PengSystems EngineerCommented:
try clearing that name from the autocomplete list.  If that name comes up - hit the delete key to remove from list.

Try sending again.

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