My ipod won't let me access my mail? It says "The user name or password for "imap.apple.yahoo.com" is incorrect" and I know my password and username are both correct.

Ive tried resetting it ive tried restoring it, my old ipod touch did the same thing and now its doing the same thing to this one. Occasionally it works but still. My internet works fine too and ive tried it on other connections too.