Company-A is already running with on-prem Exchange 2016 with hybrid configurations.
Company-B is running with purely on-prem Exchange 2016.
Now Company-B is acquired by Company-A.
Current status of Company-A Hybrid Configuration: Company-B domain verified and added as accepted domain in Company-A O365 portal.
Also Mail flow for Company-A and Company-B MX records pointed to ProofPoint Gateway for all the inbound and outbound mail flow.
Company-B will be going to decommission post movement of all the on-prem mailboxes to Company-A O365 EXO.
Hence we selected to migration type is Mailbox export to PST, upload PST into Azure Blob storage container, Mapping the PST file and Import PST into O365 mailbox. All the AD accounts will be create in Company-A local AD and sync to Azure AD via AD connect.
This will be applicable for all the mailboxes, resource mailboxes, Distribution Groups as well.
MY CONCERN: When someone from the internet sends email to email address removed for privacy reasons does it sends directly to Company-B on-prem Exchange or does it send that email directly to O365 mailbox.