Exchange with multiple domains

I work for a company that provides IT services for 5 other companies. All of the companies are on the same network and AD domain. We are currently using IMAIL and we each have our own domain email addresses that we need to be able to send and receive as. My question is whether MS Exchange can handle the different domains that each company needs (abc.com, def.com, etc.) without having to split each company to its own AD domain and mail server. I would think this would be possible but I can’t get it to work from my testing. Exchange seems to integrate with AD a little too much.