We are planning the merger of 4 companies. Each company is currently using a separate domain address. We want to utilize one of the existing domain addresses (abc.com) as the permanent domain address. The domain address abc.com is currently managing mail using a pop type service, and does not currently manage their own exchange server. The other 3 companies do. Moving abc.com onto an exchange environment will be a nice upgrade for them.
What is the best practice configuration to merge all 4 companies onto the abc.com domain address?
The initial suggestion has been to select one of the existing exchange sites to receive all abc.com mail. If the recipient address exists in that organization's AD, it should be delivered locally. If it does not exist, it will forward the mail to the next site, and so on, until the mail gets to the last designated server. This site will be designated as authoritative for abc.com, meaning if the designation mailbox is unknown, it will generate an NDR.
Would we also need to configure all organizations to all relay our outgoing mail through one server to make reverse DNS work?