Okay, I'm completely baffeled by this one. I have two Windows XP machines
(one desktop and one laptop), both with Service Pack 2 (and up to date), and
both have Outlook 2003 installed on them. I've set them both up with the
exact same account with the exact same account configuration options; the
desktop sends and receives just as it should, the laptop can only receive.

On the Laptop, when I go into the "E-Mail Accounts" configuration screen and
click on the "Test Account Settings..." button, a dialog box comes up with
two tabs. The first tab shows that the tasks "Establish network
connection", "Find outgoing mail server (SMTP)", "Find incoming mail server
(POP3)", and "Log onto incoming mail server (POP3)", all finished with a
"Status" of "Completed". Unfortunately the "Send test e-mail message" shows
a completion status of "Failed".

Flipping to the "Errors" tab I see the following message:
"Send test e-mail message: Unable to send test Message. Please verify the
E-mail Address field."

FWIW, the "Test Account Settings..." button works as it should on the
desktop machine.

When I attempt to send an actual E-Mail via Outlook 2003, I create the
message and click "Send", and the message immediately goes to the Outbox and
sits there. When I click on the "Send/Receive" button, the "Outlook
Send/Receive Progress" dialog pops up and I get the following message:
"Task 'pop.mail.yahoo.com -- Sending and Receiving' reported error
(0x800CCC0F): 'The connection to the server was interrupted. If this
problem continues, contact your server administrator or Internet service
provider (ISP).'"

Thought?