In iTunes under mail I find this: pop.att.yahoo.com (POP: my email adddress)
Under the Info tab for your iPhone sync preferences with iTunes below the Mail or Email section, is anything selected?
If an email account is selected under this section, this means the email account settings for the account were transferred to the iPhone's mail client via the iTunes sync process.
And if the AT&T store clerk manually created your email account on your iPhone using the Yahoo Mail account preset, this created the account as an IMAP account.
You have two email accounts available with the iPhone's mail client correct?
If so, and the first account is your sbcglobal.net email account, which is a POP account and matches the account settings with Outlook on your PC, what is the 2nd email account?
If the first account is your sbcglobal.net email address, which is a POP account and matches the account settings with Outlook on your PC, this means the 2nd email account must be the account the AT&T store clerk created on your iPhone. If so, delete the 2nd email account from the iPhone's mail client - the account the AT&T store clerk created on your iPhone. If the AT&T store clerk selected the Yahoo Mail account preset when creating the account, this created the account as an IMAP account and means the account is available twice on your iPhone - one as a POP account, and one as an IMAP account, which can cause problems. Deleting the 2nd Yahoo email account should resolve the problem.