It has been announced by Salesforce that from Summer 2014, Salesforce will no longer support Outlook integration with email on POP3 or IMAP servers.
They will only support email integration with Exchange servers and should you continue to use Salesforce with POP3 or IMAP email (or attempt to install Salesforce) you will receive the error message “Salesforce for Outlook has determined that you’re not connected to a Microsoft Exchange Server” and the integration will not work.
Salesforce for Outlook will only integrate with Microsoft Exchange server solutions such as Microsoft Office 365, Microsoft Hosted Exchange or Microsoft Internal Exchange Servers.
It therefore means moving your emails to an exchange environment if you want to continue to use the official Salesforce/Outlook integration. Alternatively you can use a third party integration tool such as Contact Monkey or a similar non-native IMAP/POP3 compatible Outlook plugin for Salesforce.
For more information on the news release visit https://help.salesforce.com/HTViewSolution?id=000199252