Why can I receive emails, but not send them?

[Will]

I have 3 accounts in Outlook 2007. Two of the three, I can receive emails,
just fine, but not send any out. The third account is a Windows Live account,
and it just keeps asking for my username and password, which I've checked and
rechecked to make sure its correct. But the window keeps popping up. So...
where am I messing up?

 

[N. Miller]

I have 3 accounts in Outlook 2007. Two of the three, I can receive emails,
just fine, but not send any out. The third account is a Windows Live account,
and it just keeps asking for my username and password, which I've checked and
rechecked to make sure its correct. But the window keeps popping up. So...
where am I messing up?

The biggest "Mess Up" is the lack of an error message by which we might be
able to determine a solution.