I have a client who is going through a merger with 2 other companies. Each company has it's own Exchange server and email domain. The companies would like to have shared calendars between domains. Essentially, Bob@CompanyA.com would like to share a calendar with Jim@CompanyB.com and Tom@CompanyC.com.
I know how to do this within a single domain, but not across domains.
Would it be best to take these email domains to the cloud with hosted Exchange or use an existing server at one of the companies to host email for all domains? How would one create such a setup on an onsite server?