This last week at work we upgraded several people to Office 2010, which includes Outlook. As with all upgrades you would expect at least one thing to go wrong. I thought I had hit my one hiccup when our controller’s computer did not have a DVD drive, nope I was wrong.
Apparently for a small group of people upgrading to Office 2010 the auto-complete database gets a little corrupted and will send emails to people other then the name on the screen, for example sending to firstname.lastname@example.org rather then email@example.com. This is only apparent when you mouse over the email address and see that it is a different person entirely.
Thankfully there is a simple fix to the problem, deleting the automatic completion cache as shown in this KB article. Note: This will delete all of the contents of the auto complete cache. You however can re import all the items in your address book simply by creating an email addressed to everyone and deleting it rather then sending it.