1) Technically possible, see
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2) There are improvements from BES 4.1 onwards. For foriegn domain users, when messages are sent from the device, they will be deposited in the MAIL.BOX on their home servers - hence more efficient and realiable routing. Activation wise, no issue if you have cross-certify properly and the necessary connection docs are created properly.
3) Users being in a different Domino domains does not have any bearing when it comes to OTA stuff. These are all BlackBerry/BES related not Domino.
4) Again, no bearing.