Office365 Tenant to Tenant 'slow' Migration - How to handle the directory

Slow Office365 Tenant to Tenant migration.

I want to migrate slowly from O365 Tenant A (Exchange Online) to a separate O365 Tenant B (Exchange Online).

Users at the original tenant use the DNS domain firsttenant.com, users at the the second tenant use the DNS domain secondtenant.com.

Some challenges are:
- I migrate and cutover User1 to Tenant B. Now he is using the domain secondtenant.com. How the users not migrated will send emails to User1? One idea that I have is to setup the targetaddress field of that mailbox, so it will forward the emails for that account. And in this way the non-migrated users will be able to send emails seamless to the migrated users.

- Now User1 needs to send emails to Non-Migrated users... What would be the idea here? I Can create a temporal contact on Forest B for all the accounts (and delete them gradually when these mailboxes are migrated). Another option in my mind could be to always send emails to the addresses ending with firsttenant.com until the migration is completed. But is anyone of these ideas going to break the autocomplete/cached names in Outlook?.

Any others alternative for this?
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Schnell SolutionsSystems Infrastructure EngineerAsked:
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AmitIT ArchitectCommented:
I did something similar with on premises. Where we set target address. Once users are migration to destination forest. Destination forest will be sending mail over the internet to source users. I used Mail Marshal in source domain to forward mails to destination domains, by creating contact in source domain for destination users and set the forwarding option. So mail doesn't bounce back for destination users. Same time, i set one reply to sender, for new destination email address. As external world might still try to send mail to source domain for migrated users.

Outlook, i don't remember, i did anything. I advise you to test with few test users. Then perform migration for rest. I agree, you need to migrate them slowly.
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Jian An LimSolutions ArchitectCommented:
X500 address migration will solve the auto-completion in Outlook.

User1 that migrate to tenant B always use secondtenant.com. email address
it can always write to anyone UNLESS you want it to show in address book.

If your tenant user number is low i won't bother, they still know what email address they should. but if you really need, then you can create mailcontact is the way to go, but managing them will be fun.
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