If you use more than one program to manage your mailbox, each will probably use a different name for your outbox. In webmail, you'll then see the folder listing roughly like this:

Email programs usually allow you to rename the sent mail folder, making it possible to consolidate the storage of sent messages for all applications into one common folder. For example, our webmail uses the „Sent“ folder, Mac Mail uses the „Sent Messages“ folder, Outlook uses „Sent Items“, etc…

Outlook as of version 2013 no longer supports renaming folders for storing mail, if you want to consolidate sent items in different mail programs into one common folder, you will need to adapt other applications to this default folder in Outlook.

For example, for webmail, you can change the sent mail folder in the settings:

Settings > Properties > Special Folders where for Sent you check the new „Sent Items“ instead of the original „Sent“. Sent mail from webmail will now be stored in the same folder as Outlook stores it