@ charliex: no, they don't stay in the outbox, I can send mails, but the sent mails just don't show up in the sent box.
@ rubicon: I checked Documents and Settings//Application Data/Microsoft/... found no .pst file. The "Save copies of messages in Sent Items folder"-option is checked.
well, I guess I'm gonna have to try to reinstall it and just pray
Edit:
I just archived all my mails since I wanted to make a backup before reinstalling outlook.
and all of a sudden it works again. I have no idea why, but all the mails I send are stored correctly in the sent-box.
Thanks to all of you for your help!