Yahoo! replied. The problem remains unresolved and they are still working on it.
They gave me a list of things to check. None of them help. Still, I have appended the reply to the end of this post.
So far what has worked for some of us here appears to be adding a dummy Yahoo! Mail account. It doesn't work for me though.
For those who are new to this thread. I would suggest you to please report your incident to Yahoo! in order to give them more pressure. You can report your incident at
http://help.yahoo.com/l/us/yahoo/mobile/general.html
You will be prompted to login.
-----------------------------
Hello xxx,
Thanks for writing to Yahoo! UK & Ireland Customer Care.
On behalf of the Yahoo! Customer Care Team, I would like to apologise
for the delay in answering your email.
I sincerely apologise for the continued difficulties you are
experiencing using Yahoo! Mail, and for you having to take the time to
write in again about the same issue. This problem is a known issue
imposing on other people to, I do regret. Thank you for staying in
touch.
Please read over the following link, I hope it helps:
Troubleshooting iPhone mail push
http://help.yahoo.com/kb/index?page=content&id=SLN4170&locale=en_GB&y=PROD_MAIL_ MOBILE
Please let me know if you still need assistance so I can help you
further.
Thanks again for writing to Yahoo! UK & Ireland Customer Care.
Regards,
xxx
Yahoo! UK & Ireland Customer Care