Exchange 2000 in Multiple Forests

I have an NT 4 Domain that has Exchange 5.5 running on an NT4 BDC. I also have 2 other domains that are 2000 Active Directory domains in thier own separate forests. (Each domain is in a separate forest.) These 2 AD domains don’t have their own Exchange servers. I currently have a trust set up between my NT4 domain (where Exchange is) and my 2 AD domains and they are currently able to use our Exchange 5.5 server. I plan to upgrade my NT4 domain to Windows 2000 AD and Exchange 2000. Does anyone see a problem with these other domains being able to use my new Exchange 2000 server even though they are in separate forests? I know I need to reorganize the forests but that is for a later date. Thanks in advance.