To set up IMAP on your mobile device please use these settings:
•Incoming Server - imap.mail.yahoo.com
•Outgoing Server - smtp.mail.yahoo.com
•Incoming Port - 993 (requires SSL)
•Outgoing Port - 465 (requires SSL/TLS)
•Username: full email address (for example, firstname.lastname@example.org or email@example.com)
•Password: the password you login to Yahoo! with.
For step by step instructions, see this article.