Notes has the option "Do not notify me if recipients are running out of office" .
This option does not exist if the mail sender use other mail programs or is 'outside'.
-> The domino router replace the group name (e.g. firstname.lastname@example.org) with all group member addresses independent to the OoO state:
-> If one of the recipients activated OoO the sender get a response from 'wrong' email@example.com.
1) Domino router set the (Notes option) flag for all group adresses (option in router)
2) <same> only if mail sender is outside receiver mail domain
3) Every mail group have addional settings like "Do not send if receiver activted OoO" + "... Only if sender outside receiver mail domain", ....