Pop3 email server is your incoming mail server smtp mail server is your outgoing mail server. Each email service provider have its own Incoming Outgoing (POP3 SMTP) server addresses that you can use it in MS Outlook, Eudora, Outlook Express or any other email clients.
The SMTP and POP3 server address is generally always going to be? mail. ? followed by your domain name. For ex, if my domain name is Mysite.com, my SMTP and POP3 server address would be mail.mysite.com.
If you have an ISP Connection you want to use its email box then, you have to contact with your ISP for POP3 SMTP Addresses. If your ISP provides POP3 SMTP service then it gives you the POP3 SMTP server addresses that you can use with your email client.
Yahoo's pop3 email service is not free. You have to pay for the pop email account that they will give you, which you can use your email client to send/receive mail from that address. Getting the pop3 and smtp servers from yahoo will be useful if you get one of their email accounts which supports the function. ...I believe hotmail's pop3 email is free I'm not entirely sure.