I emailed yahoo about this on friday and this is the reply I received yesterday.
Hello,
Thank you for writing to Yahoo! Mobile.
I understand that you are experiencing problems with Yahoo! Mail while
using the mail client on your Android phone. I sincerely apologize for
any inconvenience this is causing you.
At this point and time Yahoo! Mail is not supported within the mail
clients on Android phones. However, I assure you that we are working in
conjunction with the necessary parties to implement this functionality
on Android devices as soon as possible. At this time I recommend that
you remove Yahoo! Mail from the mail client on your device to avoid
additional problems from occurring. We greatly appreciate your patience
with us while we work to get Yahoo! Mail implemented for the mail client
on these devices.
In the meantime I recommend that you use the Yahoo! Mobile Mail from
your device's browser for a rich, Yahoo! Mobile Mail experience. You can
do this by entering the following URL in your device's browser:
Y! Login
Thank you for giving me the opportunity to explain the present situation
and for your patience while we work towards a solution.
Thank you again for contacting Yahoo! Mobile.
Regards,
Eve
Yahoo! Mobile Customer Care