POP3 is used for download email from a mail server.
Generally it is used to remove mail from the server and store it on your computer.
SMTP is used for sending email.
SMTP is used for both you sending email, and mail servers sending email to each other.
The incoming and outgoing mail servers for Yahoo.com are pop.mail.yahoo.com and smtp.mail.yahoo.com.
If you are in the United States and do not have a Yahoo Mail Plus
account you may need these directions for accessing a free Yahoo mail account with a POP3 program such as Microsoft Outlook
Please let me know if this information helped to answer your question.