Exchange 2000 & Exchange 5.5 (SP3)

What is the best way to integrate Exchange v5.5 and Exchange 2000 when they are in separate Org’s? My research thus far has led me to the InterOrg Utility but, this requires a prerequisite of maintaining mapped custom recipients. Normally I would use an ADC CA to replicate but, I am wary of the known issue of the AD rejecting custome recipients from and Exchange 5.5 server because their attibutes are those of server recipients.