You can do this without a macro, using System Setting's "Keyboard Shortcuts". Click on "App Shortcuts", then click the "+" button. In the dialog that appears:
- Set Application to Mail
- Set the Menu title exactly as the "From" pop-up menu item shows it, remembering that the "-" is an em-dash (⌥-). For example
My Name – m.name@example.com
- Set the "Keyboard shortcut" to something that doesn't clash with others in Mail -- perhaps ⌃S in this case?
- Click "Done", then "Done", then Quit System Settings
And you should be good -- make a new email in Mail, hit your shortcut key, and the correct account should be selected.