We just bought a company of about 100 users. We are 1000 staff large. They have their own Data Center, with their own AD which is part of a larger forest, and they use exchange 365 whereas we have on premise.
I assume the approach would be to create a one-way external trust, Security groups define access to our systems. However, how should I manage exchange 365 if they are on a tenant, eg, fin01.local? Can I simply migrate their accounts into our on premise exchange (is that doable since we are separate companies?) or do they need a local exchange member server and have it important there?