Really appreciate your help Gentlemen!!
I've read all of your replies, and have found a lot of useful information. So if one company use exchange server, and it has 2 domains then it seems to be a little bit difficult to implement BES to serve these two domains.
But, the replies also mentioned that if i have some access from one mail server to another (maybe i should make some windows' trust relationship rules first?), then it will be a chance to have my blackberry server serving these two domains.
It is also very interesting to know that BES still able to recognize other Lotus Domino mail server that is not in the same name network with the primary Lotus Domino.
I'm learning so much, again, Thank You very Much!!