If you send email from a few different accounts on your iPad, then you should know that your messages are being sent from only one of those email addresses.
But you can change them to a default address. To do so, head to the Settings app, and then to Mail, Contacts and Calendars, and then press on the “default account” option.
Then it’ll show you all the options for email addresses, where you can pick one. You can always change it at any time, and you can change them within the message itself.