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CiviMail group segementation on multisite
Overall, the mailer doesn't take into account the current users ACL. As an example it should look at the group to be mailed to (e.g. Volunteers) and then have an AND clause of the users ACL to limit the contacts it can mail to in the group. So if the group has 1000 records and my multisite domain group only has 100 records within it, I should be able to select the volunteers group and only be able to send to 100.
I also do not know if Civi can handle lots of regular groups: https://civicrm.stackexchange.com/questions/28790/email-segmentation-with-thousands-of-groups.
For chapter-based orgs, something along this is what is needed. You can see there would be tremendous redundancy if one had to re-create a group at each geographical level.
I wanted to get this down as an issue and start a conversation. I think this could be a candidate for an MIH if it was packaged as making multisite more complete.