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?
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as long as o365 find mailbox for user with @domain2.com, it will deliver mail locally

However if user don't have domain2.com mailbox, you did not defined mail routing to forward / relay that email to domain2.com at gmail side

To work with this situation, you need to change domain2.com at o365 side to internal relay and need one send connector which points emails to domain2.com through gmail SMTP server
This will ensure that send connector will relay emails to gmail for users who don't have mailbox in O365 for domain2.com,

Likewise in o365, you need to set email forwarding on migrated mailboxes (users) pointing to domain1.com at O365
ptsolutionsincAuthor Commented:
Correct the mailboxes have not been configured yet in Office 365 as we are importing 65 mailboxes.  That is why they are bouncing.  When I create a send connector, will it give me the option to only use that send connector for domain2.com?

I am currently in the Connectors wizard and at the last step Add Smart Host.  What do I enter here, the G Suite MX record?


but you also need to change domain2.com at O365 as internal relay domain from accepted domain tab in admin center
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Otherwise connector will not work
ptsolutionsincAuthor Commented:
Where in Admin Center do I change the suggestion below?

"but you also need to change domain2.com at O365 as internal relay domain from accepted domain tab in admin center"
ptsolutionsincAuthor Commented:
I think I found it in the admin center --> mail flow --> accepted domains.  I assume i have to change it from Authoritative to internal relay correct?
open exchange online admin center \ mail flow \accepted domains
yes, that's right
ptsolutionsincAuthor Commented:
Ok I have a situation.  Originally, when someone from @domain1.com would send an email to someone on @domain2.com it would get rejected because that user did not exist on the O365 account.  I created the mail routing and connector and the email was  getting redirected outside of the O365 environment and was delivered successfully.

The issue now is that i am created the new mailboxes on O365 using @domain2.com.  I did a test and anyone sending email from @domain1.com to a user on @domain2.com will get delivered to the O365 mailbox and NOT the live environment on G Suite.  How can I force all emails going to @domain2.com to send outside the O365 environment?
Why u want to do that?

Its not possible as long as domain2.com is defined as accepted domain at O365 and you have mailboxes in that domain
By default o365 checks email domain presence locally and if domain and mailbox found, deliver it and don't look for alternatives unless you set up domain to internal relay and mailbox not available in O365
This is called as co-existance

Migrated users should access O365 mailbox only

Once migration is complete you should set O365 domain to authoritative from internal relay
ptsolutionsincAuthor Commented:
I am using Skykick to migrate the 65 mailboxes.  I cant do that in one day.  Skykick helps with the migration.  you put in the source admin credentials and the destination admin credentials and it migrates all the data in the background.  its takes several days for the migration.  Users will continue to use the G Suite server until the cutover date provided by Skykick.  If users already on office365 using domain1.com send an email to domain2.com those users will not see the email until it is cutover.  what should we do?
You should follow below procedure for migration and mail flow

Since you using 3rd party tool for migration, tool can migrate mailbox in multiple pass, meaning in can migrate data incrementally, when you start 1st pass for 1st batch of users, tool migrate most of mailbox data from gmail to O365 account, user still using gmail mailbox. When you finalise migration for batch, tool set forwarder on gmail account pointing to O365 address (user@domain.onmicrosoft.com email address) of same user and convert gmail mailbox to contact and user is forced to use O365 mailbox only.
If tool does not do that, you need to manually do that so any mail received at gmail side (since your MX will point to gmail until cut over) will get forwarded to respective O365 mailbox. At this time user should discontinue gmail and must use O365 mailbox only
Batch Users should be forced to use O365 mailbox only post you finalise migration for batch. Thumb rule is that when you do shared name space migration (source and target SMTP name space is same) , user must use gmail mailbox Until you finalise mailbox migration and post mailbox migration is complete, user must discontinue gmail mailbox and use O365 mailbox only

@o365 side, you already have internal relay domain configured, so that topic is already covered

Once all users started using O365 mailbox, do MX cutover so that all external emails in out will happens from O365

That's how shared name space migration works

I hope everything would be clear now
ptsolutionsincAuthor Commented:
I understand the migration process however I think you are not understanding my issue.  I will use an example of the issue.

Company 1 is on Office 365 with domain1.com.  Company1 buys Company 2.  Company 2 is on Google G Suite and wants everyone on Company 2 to be on Office 365 with Company 1.  Created new mailboxes on Office 365 using company 2 / domain2.com.

Created connector and changed domain2.com at o365 side to internal relay as suggested.

When sender joe@domain1.com (current office 365 tenant CompanyA)  sends an email to mary@domain2.com (current Google mailbox for CompanyB) , the email gets delivered to the new o365 mailbox for mary@domain2.com.  We emails to be delivered to the Google G Suite mailbox until the migration is complete and then move the MX records.

I hope this is a better explanation of the issue.
Understood your point

Simplest solution is:
Don't allow anybody on O365 to access mailbox until you cutover, just test mailbox access at o365 side to be sure you can access mailbox post cutover.
You have only 65 mailboxes, so it don't seem too difficult, I have done this for batch of 250 users. Instruct users to access mailbox using OWA post cutover until outlook is installed and configured on their workstations

Complex way:
register one more SMTP domain at gmail side and map it with every user as one more email address
In o365 mailbox properties, set forwarder to new email address for user with option "keep copy of mailbox locally as well" for mailboxes which are still in migration process
Remove this forwarding for users whose mailbox migration is completed, else endless loop will be created
ptsolutionsincAuthor Commented:
I understand the Complex Way 100% and is an option.  I dont think the simplest solution will resolve the issue.  NONE of the Google users have access to their new o365 mailbox.

 When someone from existing o365 account @domain1.com sends an email to gmail account @domain2.com the mail gets delivered to the @domain2.com mailbox and NOT the @domain2.com gmail.
You are correct !!
I forget that domain1.com is already hosted in O365 and they can send emails to domain2.com......
Actually there is one more option but I don't know if you already started migration

1 do not register domain2.com with o365 but create user accounts in O365 with onmicrosoft.com ID and grant them mailbox
2 do mapping between gmail ID and O365 onmicrosoft.com ID
3 migrate mailboxes in multiple passes from gmail to O365

4 Once all mailboxes complete initial migration, do cutover. In cutover add domain2.com to O365 and add domain2.com email  addresses to onmicrosoft.com users as primary address and point you domain2.com MX to O365. finally run final pass of migration
5 disable google mail and instruct users to use OWA until outlook gets configured.

Now you don't need to worry of domain1.com users

I have used this process with BitTitan tool few years back

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