I got tired of not receiving yahoo mail when I was using wifi. Here is a way to forward yahoo mail for free to gmail. I also like receiving all my mail in one inbox. Here is a link to the original androidforums thread.
http://androidforums.com/motorola-droid/30776-forward-yahoo-mail-gmail-free.html
http://androidforums.com/motorola-droid/30776-forward-yahoo-mail-gmail-free.html