From my understanding, you have 2 email accounts on your device, one from MSN and one from Yahoo. But, you want the sending of all emails from your phone to solely come from MSN by default. In which case, you can open up Settings.app, and scroll down a little to find Mail,Contacts and Calendar, and scroll down towards the bottom until you see "Default Account". Tap on it, and from there, you'd be provided with a list of your email accounts that are "registered" or logged-in by your device, and you can choose your MSN address/account there.