Several readers are reporting an issue where Yahoo! email accounts do not push messages to the iPhone.
Yahoo! Mail accounts are the only type of accounts currently accessible by the iPhone that offer push services. This, in theory, means that you will be instantly notified when a new message lands in your Yahoo! account mailbox. The service works by routing email through imap.apple.mail.yahoo.com, a mail server designated specifically for the iPhone by Yahoo!. Other IMAP and POP accounts accessed by the iPhone must rely on time-interval checking (ranging from every 15 minutes to every hour, toggled in the Mail pane of Settings) or manual checking.
Push mail is supposed be offered with free Yahoo! mail accounts.
However, Yahoo! mail is not "pushing" for some, only appearing in the inbox when the user manually checks, or during a time-interval checks.
There are a few potential workarounds for this issue:
Reset the iPhone Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears.
Delete then recreate account Go to the Settings application, tap Mail then tap your Yahoo! account and tap Delete Acount. Then go back to the Mail pane of Settings and re-establish your Yahoo! account. You may need to reset your iPhone again after doing this.
Create an all-new account In some cases, the only option is to create an entirely new Yahoo! account at mail.yahoo.com. Some accounts don't, for whatever reason, appear to be working properly with push services.