Assuming of course that you have the paid Yahoo! Plus email that enables you to use POP and SMTP error 550 means you need to enable server authentication in your mail client.
This is right from Yahoo!'s support:
said by Yahoo! :
Incoming mail server settings
POP server: plus.pop.mail.yahoo.com
Outgoing mail server (SMTP) settings
SMTP server: plus.smtp.mail.yahoo.com
Account Name/Login Name: Your Yahoo! Mail ID (your email address without the "@yahoo.com", for example, testing80)
Email Address: Your Yahoo! Mail address (for example, email@example.com)
Password: Your Yahoo! Mail password
If you still cant send Yahoo! Mail from your mail program, you may be accessing the Internet through an ISP that is blocking the SMTP port. If this is the case, try setting the SMTP port number to 587 when sending email via Yahoo!'s SMTP server.
Edit: Bresnan does block the default SMTP port 25, haven't ever used 465 so can't comment but I do use 587 quite frequently (not Yahoo!, though) and it works just fine.