By default Microsoft Outlook 2010 has a feature enabled which automatically creates Outlook contact for people who you have sent or received e-mails from.
In your folder list you will see a contact address book called Suggested Contacts which accumulates these addresses
If you use an iPhone with Microsoft exchange it will automatically synchronise this contact list meaning you may end up with hundreds of additional contacts in your phone that you do not want. In order to clear these delete all the contacts in the Suggested Contacts folder.
Then in outlook options –