David,

first thanks for commenting, my aim is to understand your
need better in order to help creating a cool solution. ;)

The question I had was: 
How does sendmail get this email away from your machine.
I mean: If sendmail can send out emails to another machine without need
to authenticate itself, then roundup would be able, too.
My conclusion is that sendmail has the necessary credentials.

So you are calling sendmail which probably is a suid root or sgid root
application and then uses it higher access rights to read the saved
credentials. 

If this is the case, you could configure your sendmail to just listen
to a local port without authentification and accept emails from roundup
from there. 

Best,
Bernhard