Yes, right. Minimum 1 per sender.
From RFC 5230: Sieve Email Filtering: Vacation Extension
So the vacation notice is not the right function for your requirement.
This is actually a case for an Mail filter. But I don’t see any possibility of that at the moment.
I understand the requirement and will discuss this with Birgit. We have to see if this is technically (EGroupware/sieve) feasible.
Of course, when dealing with such things, one must not forget the problems that such behavior brings with it: mail loops/mail bounces.
I will get back to you.