Since I have removed all other email accounts on my iPhone and just use Yahoo, push seems to work much, much better. I only wish I could change the reply-to address that is uses as I hate having a @yahoo.com account.
Not sure why Apple doesn't do push with their .Mac accounts, although maybe this will be coming soon.
I don't think adding or deleting accounts is the solution. I have done this over and over and I have a Yahoo account that just will NOT receive email.
What is weird is that I created a brand new Yahoo Mail account, added that one (on top of 3 Gmail accounts, a Yahoo account and my .mac account) and that NEW Yahoo Mail account can send and receive.
I did some tests today and it seems, for me, that my .mac account is the fastest at sending and receiving.
I tried using Yahoo Help but it is WORTHLESS and I was eventually forwarded to some ATT site that has NOTHING to do with my issue.
I really may be done with Yahoo, especially with that BS advertising they put as your signature..