Consider the following scenario:
You deploy Microsoft Exchange server and Microsoft Lync Server 2010 in an environment.
You remove a user's account in Active Directory directory service.
You set the user's primary Simple Mail Transfer Protocol (SMTP) address as your secondary SMTP address. This prevents messages that are sent to the user from being rejected.
In this scenario, the user name is still in Lync 2010 when you search for it.
To resolve this issue, install the following cumulative update:
2496325 Description of the cumulative update package for Lync 2010: April 2011
After you apply this update, you can configure the EnableContactMergeUsingProxyAddress setting to disable merging contacts that are based on secondary SMTP addresses.
To work around this issue, you can manually remove the email address of the user from the Outlook contacts.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.