ODM O365 to O365 migration and Azure Information Protection Encrypted emails

EDIT:- "Which is difficult because it can* only be tested after Domain cut-over"

There is not a lot of people with that service enabled AND doing a migration. That would be why Support said testing is the easiest way to validate. Just simply migrate a mailbox with encrypted messages, Logon and try and open the message. 

The issue is access to the encryption keys, the target users will not have access to the encryption keys. So while the message can be migrated, it cant be decrypted without the encryption key. That I assume is tied to the user and not the UPN, or Domain you are cutting over. 

There is not a lot of people with that service enabled AND doing a migration. That would be why Support said testing is the easiest way to validate. Just simply migrate a mailbox with encrypted messages, Logon and try and open the message. 

The issue is access to the encryption keys, the target users will not have access to the encryption keys. So while the message can be migrated, it cant be decrypted without the encryption key. That I assume is tied to the user and not the UPN, or Domain you are cutting over. 

That's the thing, Support are suggesting that once the original email address is attached as an Alias to the destination mailbox, the encrypted messages should be readable after migration. To test that I've to cutover the domains. Simply migrating the mailbox isn't enough.