This workaround applies to Microsoft Outlook 2003. I have not tested it on any other version of Outlook.
In my case, I found that after I upgraded Zone Alarm to the most recent version it rendered Outlook's Send/Receive useless for certain email accounts. To get around this, I found that iSafe.exe was the problem. While iSafe.exe was running, the Send/Receive error occurred.
To fix this, end the iSafe.exe process. For more on what iSafe.exe is, you can google it. For now, just know that this is the culprit. You can end the process by ending the process in Task Manager. To get there, right-click on your taskbar then choose Task Manager. Click on Process tab then sort by Image Name to make the list manageable. End the iSafe.exe process.
Perform a Send/Receive (no need to restart Outlook) and it should work.
Best of luck & hope this helps someone,
Mike