rowan.bradley,

Using multiple accounts in Outlook 2007 and in Outlook 2010, message are sent to the active account sent items folder, most times.

Lets say E1, E2, and E3 are your Email accounts. If a message is sent to E2, it is shown in the E2 inbox. If you reply to this message, you have the chocie to send as any of the three accounts. Note: outlook will ALWAYS default to the defaulted account that has been set ion your account settings.

With this being said, you want to answer the message using he E2 account, this wil place the message in the E2 send folder.

If you Answer the message either with a different account or as a different account after answering it with the correct account the message will be moved to that account send items folder as you are using that accounts SMTP information.

The best way to confirm this theory is to select one of the messages you see out of place and view the full header files of the message to see if you replied at some point with a different account.

Hope this helps.