I have two active directory domains (C1.com and C2.com) in separate cities that are not in the same forest. C1.com and C2.com are synced to separate Azure subscriptions using Azure AD Connect. I use the C2.com domain for user authentication in that Azure subscription and for website addresses.
I have a single Exchange server in C1.com that accepts e-mail for C2.com as an alias to users. (me@C1.com has a secondary e-mail address of me@C2.com). There are also a couple of "e-mail only" domains (no AD) that are used as aliases. This works fine and has for many years.
I now want to migrate my Exchange server to Office 365 E3. I logged on to the Office365 portal and added my C1.com to my account using the txt record for verification with no problems. I also added the e-mail only domains. I have not modified the MX records yet.
When I add the C2.com domain I get the message "We have confirmed that you own C2.com, but we cannot add it to this tenant at this time. The domain is already added to a different Office 365 tenant: UserC2onmicrosoft.com."
All I need is the ability to route e-mail into Office365 from C2.com. I don't really care about the AD users in Office365. I saw this article https://docs.microsoft.com/en-us/azure/billing/billing-subscription-transfer, but I am not sure if it will accomplish what I want. Since we have client production workload running on both domains in Azure, I need to understand the ramifications of any change I make before I start reconfiguring things.
Thanks in advance for any help and advice.