Integrating your e-mails with iPhones

The first thing you need to do is enable checking email from an external account in your yahoo mail settings. Instructions are here:

https://help.yahoo.com/kb/index?page=content&id=SLN3255&actp=search&viewlocale=en_US&searchid=1372387650987&locale=en_US&y=PROD_MAIL_ML

Then, you can setup PoP3 access in inbox.com so that it will check your Yahoo mail account. Instructions are here:

http://www.inbox.com/faq_email.aspx?id=1734

Good luck!

Email account setup with Mail is easy. If you're setting up an email account for the first time on iPhone or iPod touch, simply touch the Mail icon on the Home screen. If you're adding an additional account with software version 2.0 or later, touch the Settings icon on the Home screen, select Mail, Contacts, Calendars and select Add Account.

If you're setting up a Microsoft Exchange, MobileMe, Gmail, Yahoo! or AOL account, touch the icons for those services on the Add Account screen and enter your account information. If you're setting up an email account from another provider, select Other. In software version 2.0 or later, Mail can automatically setup many popular POP and IMAP email accounts using just your email address and password.1 As Mail looks up settings that match your email account you may see a note appear that will describe any limitations you may experience in using your email account. If Mail cannot find the settings needed to setup your email account, refer to the documentation from your email provider on how to manually setup an email account.

Syncing email account settings to iPhone or iPod touch
You use iTunes to sync your email account setttings to iPhone or iPod touch. iTunes supports Mail on a Mac, and Microsoft Outlook (2007 or 2003) and Outlook Express on a PC. See article "Syncing iPhone and iPod touch with your computer" for further details.

Note: Syncing email account settings to iPhone or iPod touch copies the email account setup, not the messages themselves. Whether the messages in your inbox appear on both iPhone or iPod touch and your computer depends on the type of email account you have and how it’s configured. You may need to enter your password on the device after syncing account settings.

http://www.youtube.com/watch?v=644kJKm2l30

if you are trying to setup the yahoo mail account as a pop3 account on your phone then you would require a premium account from yahoo and only then is it authorised from the yahoo server until then only the settings can be put on the phone

try a google account just to be sure or for that matter any other mail service which supports pop3 access