It could be a profile issues. I would start by recreating out Outlook Profile. I asssume you are not using an Exchange Server and using POP3 / IMAP instead. In which case, I would do as follows:
1. Take a Backup of all my Mail by either copying the PST File to another location or using the File | Import Export utility to Export all my mail data to a PST file.
2. Go to Start | Control Panel and Double Click on the Mail Icon
3. Click Show Profiles – (Don’t delete the exisiting profile. Lets create a new one first and see if it works)
4. Click Add and give it a new profile name. Go through the wizard and create a new pop3/imap profile. You need your Outlook account settings for this including passwords and mail server addresses.
5. Once you have completed this, you will go back to the General Tab of the Mail Setup. At the bottom, select the new profile to launch Outlook with.
6. Test the Send and Receive now and let us know if it works.
7. If it does, you may have to rename your NK2 file to the new profile name which is located in C:\Documents and Settings\<i>your username</i>\Application Data\Microsoft\Outlook. The NK2 file stores a list of cached email addresses which you use when typing in someone’s email address in the To: bar. It trie to autocomplete the email address for you. A very useful thing indeed. But first backup the old NK2 file and then rename to match your profile name you selected in point 4. above.
You should be ok now. If not, perhaps you might need to re-install Outlook but then at least you have your PST backup with you as per the above.
Hope this helps