We have Office 365 with ADFS for our domain (let's say domain.org) currently. We're considering converting about 150 users to it, all of whom currently have email via an Exchange server on a different email domain (otherdomain.com). We want these users set up to where they'll have a domain.org email address, but if someone emails the old otherdomain.com address it will reroute to them. I know how to set up the aliases and have read that we would need otherdomain.com as an available UPN suffix.
Here's where things get tricky. Otherdomain wants to continue to have their own Active Directory and authenticate email via a Proxy. Are there any configuration steps we will need to know about or limitations of doing something like this? They will eventually be on the same Windows domain, but it is going to be a LONG time. They want to get email up and going as soon as they can on Office 365 to utilize the eDiscovery features and larger mailbox storage size. Has anyone ever had a similar experience?