Migrate from exchange 2010 to exchange online Office 365


I´ve some mail account on premise exchange  which I´like to migrate  to exchange online. At this point.. easy.
I choose one email account, mine, and  

-create the same account at exchange online.
-validate the domain
And now, I have inserted a 0 MX record at DNS. There are two records, 0 value for office 365 and 10 for my existing enviroment. The problem is when I send an email to an email account which is only defined at on premise server.. an error is reported.

How to accomplish this movement without broking the service for non migrated accounts?

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Ravi AtreyCommented:

You have made an mistake. MX decides the priority level which mail server to choose first. As your on prem Exchange server is live and you are just testing O365, hence in this case, you must set MX priority 0 to your on prem and 10 for your O365.

This will route all your mails to on prem exchange.

Now, as you are testing stuff on O365 by creating your own mail id on it, you can send mail from O365 mail id to any external domain like gmail, yahoo etc except your own domain's mail ids as if you would try, it will bounce with showing that "user doesn't exists". Hence on O365, you must create contacts for all mail ids which exists on your on prem and also need to create an Outbound connector to route local delivery mails to your on prem exchange.

Ugo MenaCommented:
You can use powershell to migrate users one at a time, or multiple users in a batch.


The duration of your migration will vary based on the number of mailboxes in the batch, the size of each mailbox, and your available network capacity.

All in all, the on premise users end up with a mail forwarded account that allows the Exchange 2010 server to keep sending and receiving mail for all users.

It works well and allows you to maintain a hybrid environment is you so desire.

MX records will determine which mail server it tries first and next if the first is not available.
heze54Author Commented:
0 mx -> on premise
10 mx ->office 365

I migrated all my emails from on premise to office 365 and is ready to go to production enviroment but.. one doubt I have more mail users/account running  exchange on premise  but i´m going to delete  my account at on premise.

If I try to send email the mx points to  on premise eviroment and an error is shown from, for example, gmail but other on premise users must work in that way.

How can I tell on premise exchange to check unknown email accounts try to check them at office 365?

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Ugo MenaCommented:
If you are going to keep your on premise exchange receiving email for the domain ..  Mx set to 0., you will need to use the alternate domain on o365 to fwd to users you have already migrated to the cloud. It is usually yourDomain.OnMicrosoft.Com.

If using the powershell commands, the migrated users should have had a contact created onpremise for their onmicrosoft.Com account which the onpremise exchange account will now fwd mail to.

You will need to adjust your spf record to allow for both hosts to send email for your domain.
heze54Author Commented:

I´m going to keep my on premise exchange because there are still user not being migrated..Could you explain better the mydomain.onmigrosoft.com option?

Ugo MenaCommented:
When you sign up for office 365, your domain is transitioned from your provider and Microsoft will create another domain that is your domain.onmicrosoft.com.
They will use that domain to begin the domain transition to their cloud. Both domains will always remain active and will use, or not use based on mx records to receive email.
Log into your account via owa on office 365 and then go to add an alias to your account. You will see the on Microsoft domain and user account info there.

If you want to keep a hybrid without federating active directory, you can use this little known transition domain to keep your on premise receiving your regular domain email and allow office 365 users to get email fwd to onmicrosoft.com account.

You should have migrated users stop using onpremise login to avoid confusion.

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heze54Author Commented:
Nice post!!
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