Assuming you are using a later version of Microsoft Outlook (e.g. 2002,
2003, or 2007), then I would guess that your <profilename>.SRS file is
corrupt. The .SRS file is located at \documents and
settings\<userid>\application data\microsoft\outlook. All you have to do is
rename/delete that file while Outlook is closed. Next time you start
Outlook, you will have to setup your send/receive settings (ctrl + alt + s)
again.
/neo
PS - The folder "Application Data" is a hidden/system folder, therefore you
must configure Windows Explorer to show you these types of folders in order
to complete the above.