In general, this means that some part(s) of your SMTP server setting(s) within the account don't match the requirements at Yahoo. The target SMTP server isn't the correct host name, the port is wrong, whether or not SSL is required is incorrect, or the username and/or password is incorrect.
The SMTP server address is smtp.mail.yahoo.com The user name us your full Yahoo! Mail email address including the domain "@yahoo.com" or whatever Yahoo domain you're using. The SMTP server password is your Yahoo password. The TCP port is 465 (one of the default ports in OS X Mail.app client), and SSL/TLS is required.
This site contains user submitted content, comments and opinions and is for informational purposes only.
Apple disclaims any and all liability for the acts, omissions and conduct of any third parties in connection with or related to your use of the site.