Yahoo's small business mail service does not support IMAP except for mobile devices - see:
http://help.yahoo.com/l/us/yahoo/smallbusiness/bizmail/pop/pop-39.html
Yahoo warns that emails may be deleted if you attempt to use IMAP for non-mobile devices.
You can use POP but this only transfers mail from the server Inbox to your client Inbox. Folders are not synchronised. Mail is normally stored on the client computer and not the server computer as with IMAP. I prefer to use POP but most people use IMAP. I keep mail on the server for 122 days (4 months) and bcc all outgoing mail to myself. This keeps my several computers synchronised. I can still use webmail when away from home.
Small business POP settings are here:
http://help.yahoo.com/l/us/yahoo/smallbusiness/bizmail/pop/pop-43.html