I’m hoping feature requests are allowed here!
It would be really useful to be able to specify (in addition to subscriber lists for sending) lists to NOT send to.
A simple case would be where one list is people who have bought product A, another is people who have bought product B. Some people have bought both. I’d like to be able to mail everyone who has bought product A about product B - but not if they have already bought it and are therefore on the list for product B.