If this does not work for you... add another account to your iphone (in my case I did the account for my work...but I left out the information to pull the e-mails...I only put the information to send them.).
It will give you an error if you try to go view that e-mail box...on the iphone...but I dont since I know its a dummy.
Now when you check an e-mail using the yahoo box...and hit reply...it will let you click the from button...(since there are now more than one e-mail account on the iphone) just change it to the dummy "out-going" account address listed). It will send it using the "out going" information on the dummy account. Walaaah.
Hope that made sense?
It will give you an error if you try to go view that e-mail box...on the iphone...but I dont since I know its a dummy.
Now when you check an e-mail using the yahoo box...and hit reply...it will let you click the from button...(since there are now more than one e-mail account on the iphone) just change it to the dummy "out-going" account address listed). It will send it using the "out going" information on the dummy account. Walaaah.
Hope that made sense?
First I used the Mail setup to add and account, then I used the Yahoo Button. I set up my yahoo account completely normally, I put in my name@yahoo.com address and password.
No problems.
Then after I confirmed the yahoo account was working, I went back into setup and changed the address field that had my yahoo account to be name@mydomain.com, I didn't do anything to any other fields. Now my emails I send from the iphone all go through Yahoo, work via push, but the recipient sees them as coming from name@mydomain.com Perfect!!