Problems with Yahoo's smtp server?

I just moved to Damascus, Syria and have Syriatel as my ISP over a 3G network. I have a Yahoo Plus e-mail account but I am unable to send messages via Outlook. I receive messages fine and I know the problem is with Yahoo's smtp server because I was able to successfully send a test message to my gmail account via their smtp server - thus it isn't an ISP problem. Does anyone have a recommended solution or am I just wasting my $19.99 on a Yahoo e-mail pus account?