I have 3 public email domains that I need to merge due to a corporate merger. The domains we will call x.com, y.com and z.com. All email for x.com and y.com need to go to z.com moving forward. As I understand it, I will need to go to the MX records for the x.com and y.com domains and redirect them to the public IP for z.com and then setup a receive connector to accept x.com and y.com traffic to pick up legacy traffic in the interim. I also believe that I need to create an email address policy to add the new domain suffixes to the migrated users.
What else am I not thinking about?
You have the right idea about pointing the MX records to z.com public IP for used for mail. The next thing you need to do is not create a receive connector but create an Accepted Domain for x.com and y.com in z.com Exchange environment.
From there you will then create EAP eamil address policies for x.com and y.com. These policies will apply to your users from there respective domains based on specific parameters that you specify OU, or Location etc.
Once that is done you will be able to receive and send as all 3 domains from a single Exchange server.