Outlook 2007 with ypops does not send mail

Question

i am new to outlook, and i wanted to use it for my yahoo mail (free) account... i browsed the net and found that a combination of outlook and ypops can do it.i configured ypops and according to the instructions set the incoming port to 1101 and outgoing (smtp) port to 251.i am able to receive mail, but not able to send any.i get a reply from the administrator with the following test:

Your message did not reach
some or all of the intended recipients.

Subject: hi

Sent: 2/14/2008
10:06 AM

The
following recipient(s) cannot be reached:

'[e-mail]' on 2/14/2008
10:07 AM

554 Mailbox unavailable.

How to solve this...please help methanks in advanceSandeep