I recently had a customer who acquired another company. Let's call that "newcompany.com".
The new company was currently using hosted Lotus Notes, so my customer desired to switch them onto the existing Kerio Connect Mailserver. That's easy enough to do - you just "Add a domain" in the Kerio Admin browser and add users to that domain.
You could do this by putting up a whole new mail server at the new company, but you can also do it this way. All you need to have is sufficient user licenses to cover the people being added. You could also make "newcompany.com" be an alias of your existing mail domain and again just add the users. That's often done as a temporary measure where "newcompany" addresses will eventually disappear.
Note that Kerio also supports distributed domains, which might also be useful in this sort of merger or acquisition.
In this case, the "newcompany.com" adresses would remain for the foreseeable future, so adding a separate domain makes sense. Note that this is not a separate mailserver - it appears to be to the outside world, but is administered as one server locally. No additional costs are incurred other than any additional user licenses needed.
However, there is a small gotcha. The minute you add the "newcompany.com" domain, local Kerio users can no longer send email to people who are still using the hosted Lotus Notes server - they won't be able to do that until all the users are added and the MX record changes have been made.
Integrating a new company like this may require relocation of personnel, new group memberships and so on. The original idea was to do the setup work over the weekend and do the actual switch late Sunday afternoon, but it was still necessary to be able to communicate with "newcompany.com" during that work. How can that be done?
Kerio makes it easier by allowing you to easily rename a mail domain. They can create the domain with a fake name - say "foobar.com". Users can be added, groups and aliases created without affecting "newcompany.com". When they are ready to do the switch, they will rename "foobar.com" to "newcompany.com" and everything is ready to go.
Note that groups, aliases and even mailing lists and calendar resource schedules are renamed to "newcompany.com". There are a few caveats: mail filters containing "foobar.com" would not be fixed up (if you needed to create them ahead of time, use "newcompany.com" instead) and distributed domains require special handling, but most of the work is accomplished with this simple renaming and a restart of the server.
See 7.8 Rename Domain in the Kerio Connect Administrator's Guide for more information.