How do I configure SendGuard to always prompt me to select an account when sending emails?

You need to activate Account Guard first.
  1. Open Outlook and go to Inbox > SendGuard Outlook Addins > SendGuard
  2. Click Settings to open the SendGuard Settings screen.
  3. Go to the Account Guard tab > Enable Account Guard
4. Once this is enabled a prompt will appear for you to select an account each time you send an email.