Outlook 2002 Send/Receive error

I am having trouble with manual send/receive. When I click the send/recieve button in Outlook or press F9, I get a message that just says “The operation failed”. I have my outlook set to automatically send/receive emails every few minutes, and that is working. I am able to send out messages and I am recieving them, but I just can’t do it manually. This isn’t an immediate concern since I am getting my emails, but I don’t understand why it’s not working when I do it manually. Any help?