Can you give us more details?
POP3 settings, servers for incoming and outgoing mail, ....?
Beware that Yahoo Mail doesn't support IMAP.
From
emailaddressmanager.com:
Yahoo! Mail offers standard POP3 access for receiving emails incoming through your Yahoo mailbox, by using your favorite email client software. To setup your email client for working with your Yahoo account, you need to select the POP3 protocol and use the following mail server settings:
Yahoo Incoming Mail Server (POP3) - pop.mail.yahoo.com (port 110)
Yahoo Outgoing Mail Server (SMTP) - smtp.mail.yahoo.com (port 25)
Found via
Google.
PS:
1. at deriko100: strange, I used Mail to access my Yahoo Mail before, and I have the free variant and not the Plus one.
2. you could use Google Mail to access Yahoo Mail, and then use Mail.app for using the Google Mail account (via IMAP), but if that is possible, so should using Mail.app and the Yahoo account be.