The fields in the "Mail Server Settings" panel should be arranged in this order:
- Enable SMTP?
- SMTP Auth? (when that's checked, enable the username and password fields)
- Start TLS?
- SSL Trust
- Email Prefix
Rearranging and reorganizing the fields in that way should make a more logical flow to changing settings in the UI. These changes would need to be made to the /setup page as well.