John Simpsons patch already includes the patch for using smtproutes so all we need to do is in the smtproutes file you can add a couple of extra arguments of a username and password after the host. For example see the example below user password

then it will authenticate with the remote host to send email. With just the colon ':' at the start of the line that's the 'default' route for mail. If it was just for a particular domain, then put that in front of the colon.

Special thanks to Derek for documenting this.