You can choose an alternate 'from:' account in Boxer as long as you have other accounts connected. To do so:
1. Tap in the 'To' field. You will see the favorites icon show up to the right.
2. Next to the favorites icon, and just below the 'Send' button is an arrow that will open up a drop-down.
3. Tap on the arrow to open the menu.4. Select the 'From' line, and you will be presented with a list of your email accounts to choose from.