You probably have a problem in your SMTP settings in Pocket Outlook. See these posts for suggestions:
Trouble Sending Comcast Email
Wifi, Port 25 and Port 587, and SMTP
Note: Your Yahoo configuration is likely not the same as the other providers in the links, which are provided as examples. Check with Yahoo for the proper security settings.
For troubleshooting Yahoo's POP3 connection, see here:
http://help.yahoo.com/help/uk/mail/pop/index.html