Have you tried activating with the email address
? If all users have both email addresses and the dept1 accounts activate fine but not dept2, I would see if that would work. I have serveral email addresses in exchange and I have no issues getting mail sen to any of the address address.
I could be completely off base but I don't think there is any registration that takes place during the communication between BES and RIM and I beleive the communication is only one way, BES --> RIM. I think there is an order or something that this communication happens becuase if you have two seperate installs of BES within your network and enable both using the SRP, RIM stops accepting communcation from that SRP. That leads me to believe IP address isn't a factor. It is just and identifier type thing I guess.