The Mail app for iPad lets you add multiple email accounts from multiple email service providers, but if you're not familiar with the iOS 4 operating system, changing accounts can be confusing at first. Apple lets you set a default email account in your Settings app, which will be the automatic "From" email address when you write new email messages. However, you can also select different email addresses to change between accounts in the open message window.
Tap the "Settings" app icon on your iPad to change your default email account.
Tap "Mail, Calendars, Contacts." Scroll down to the "Mail" heading and tap "Default Account."
Tap an account to select it as your default.
Tap the "Mail" icon and then tap the "New Message" icon.
Tap the "From" field to change between email accounts when sending a message. Tap the email account you want to use.