Having multiple email accounts on your iPhone is convenient, especially now with a unified Mail inbox in iOS 4.2. When sharing a link or photo though, Mail automatically chooses to send it with your default email account. Tapping on the sharing button in Safari, Photos or other third-party apps brings up the following menu:
To change your default email account and make it active for sharing links and photos, begin by opening Settings from the home screen.
Scroll down and tap on Mail, Contacts, Calendars section.
From the Mail preferences window, tap on Default Account.
In the next window, a list of your attached email accounts will appear. Tap on the email account you would like to use as your default.