Please note that Outlook 2010 is not a supported IMAP client.
When using Outlook, the Sent folder is not synchronized.
Instead, the emails are stored locally to Outlook in the personal Sent folder.
In order to have a reference to these emails, you must create a folder within Outlook (with a different name than "Sent" - example: "SentFromOutlook").
Then, within Outlook, you must specify a rule to have all Sent items stored in the folder created in the previous step.
Please note that the Sent folder (when viewing through the web-mail interface) and the newly created folder used from Outlook will contain different Sent items based on where the email was sent from.
Both folders will be visible when viewing through the web-mail interface, but Outlook will never synch with the Sent folder.
This is the expected behavior with Outlook.
In Outlook 2007, when attempting to send the first email, the following message is displayed:
"You have not chosen a folder to save copies of your sent messages for
this e-mail account. If you do not choose a folder, copies of your sent
items are only saved locally in the default Outlook Sent Items folder."