Your requirement is a tricky one, especially as you presumably want to consider CC recipients as well as TO recipients. The challenge is writing a condition that allows you to identify recipients that are
not sent to "So_&_So@...".
The trick I think is to count the number of recipients. Using standard sieve, code of the following form is supported:
Code:
address :count "gt" :comparator "i;ascii-numeric" ["to", "cc"] "1"
The approach I would use is move all messages sent to the relevant alias with more than one recipient to the "service" folder. Next I would provide the appropriate rules to move messages sent to the relevant alias (now known not to have other recipients) to the appropriate folders. I would have a last rule that handles remaining messages from "service@XXX_XXXX.com".
I do not have time to create and test the solution, but hopefully the above will help get you started.