Multiple domain email addresses to a single 365 mailbox

jcorrigan used Ask the Experts™
We have several different domains on different GSuite accounts.  Our original primary domain is on Exchange.  I recently migrated that account to Exchange 365.  Now i would like to move all the gsuite accounts to my 365 account.  the problem is that userA has a 365 mailbox and also has 2-3 gsuite mailboxes.  what i would like to do is assign the email addresses to the primary 365 user, then point the MX records to 365 so that any mail sent to that users gsuite account would route to his 365 mailbox..  not 100% sure that will work but if it does, the next question would be, how do i get each users mail from their gsuite account into their 365 mailbox.  Maintaining all these accounts across two different platforms is causing issues for users with calendars, etc.
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Step1. Go to O365 Admin pane and add all the domains. If you don't see the Domain part on the home, just add it

Note: when you add the emails it will ask you to modify the MX records for all the domains. You can do that just to add the domain and then change it, or just leave it to o365.

Step2. Go to the Users/Active Users and select a user, then on the address, you can add more addresses the good point is that you will be able to add the alias matching the domain(s) you want.
By adding alias it's not the same as adding "a new Account" because all the emails sent from that account will be matched to the User Principal Name of the user (basically the email address from the one they log into the email or O365 the account).

Sidenote: If they want to have the "FROM" part of the address to match a specific this can only be done by creating a NEW MAILBOX, which would need an O365 license.

Step3 Migration: to migrate you can use IMAP

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