You're going to have to talk to whoever maintains your network.  One
possibility, if you have the ISA proxy, is that Outlook needs to be added
to the list of programs allowed - the default configuration of ISA is to
disallow Outlook.

Besides other responses, I discourage you to use your yahoo private mail
account in the company you work for, unless you want your boss to read all
incoming and outgoing messages from that account. In many companies the
e-mail is strictly monitored by a special program between your computer and
and the Internet - without you being notified. For private correspondence it
is safer - from the continuity of the employment contract - to receive
private e-mail at home or in an Internet-cafe.

Vlad