Migrating G Suite to an an existing Office 365 account
I am in the beginning stages of a G Suite to Office 365 migration. I have an issue that hopefully someone can guide me. The end user already has an existing Office 365 account with other domain names (domainname1.com). I added the current G Suite domain (domainname2.com) so I can start creating the mailboxes but whenever someone from domainname1.com tries to send an email to domainname2.com the email gets rejected. I guess it is because it sees the domain name locally / on the same O365 server and not looking at the DNS records as the MX record is still pointing to their live environment with Google. Is there a way around this so I can re-add the domain, start creating all the mailboxes and migrate the data?