I have my Yahoo AT&T mail setup on Outlook at home (with POP & SMTP settings) it works fine. I sync'd up my iPhone 3G using iTunes - as it captured the email accounts I have on Outlook. On the iPhone it downloads the messages fine, so the POP3 settings work, but when I try to send email, it tells me "Cannot Send Mail" & " The Sender Address was invalid".

Currently the SMTP is set to "smtp.att.yahoo.com". I have also tried "smtp.mail.yahoo.com"

Any ideas?