That's an interesting observation about leaving the SMTP field blank and having it send from the default account. This is almost a feature we can leverage. Assume for a moment that yahoo push mail actually works
I have my personal email forwarded to my yahoo account so I can use push. I'd like to reply via my personal email, however. Since the iPhone mail app lacks the ability to fill in the "reply-to" field of email headers, this might be a workaround.
Here's the problem: when you setup a yahoo mail account on the iPhone, it just asks for your login and password... So it automatically sets up the SMTP. If I add the account as a custom POP account, will it still push?