You can choose the email account and from address or alias by tapping on the from address when editing a new email.

You can edit the 'From' address for your email account in the outgoing mail server screen. Open the edit email account settings screen and tap on 'Outgoing Mail Server'. (View)

You can also add multiple extra email address aliases. In the outgoing mail server screen, tap on 'Email Aliases' to edit the alias addresses.

Tap on the "+" button at the top of the screen to add a new alias.
When you are editing an alias, you can simply add a new address. You can also choose to add a name for the alias, otherwise the name will be the default account from name.

You can also choose a default signature to use for the alias (View)