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We have a secondary mail/web domain that has its own mail running through postfix and it is just being forwarded to the primary mail/web domain on exchange and O365. We want to add all of the email accounts from the secondary domain to the users of the primary domains to remove this "relay".

I want to try to minimize interruption so my firs thought is to add the user alias for the secondary domain. This is not syncing up to O365 and I suspect it is because O365 has not been made aware of the domain. Correct me if I am wrong here, if I add a domain to O365, exchange will stop sending out to that address and try to deliver it internally? Of course this would be only internally originating email, but that will cause some issues as we would get delivery errors.

That being said, is it save to expect that if I sync all the alias addresses over to our on-prem server, which will not sync, then add the domain to O365 that the sync will start picking up those alias addresses or do I need to add the addresses after the domain add?

Bryan
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I'm not quite following you, but here are a couple of pointers:

  1. You need to add and verify the domain in Office 365. This in and of itself will not change where the email is being delivered.

  2. You need to add the aliases for this domain to all of your users in Office 365:

janedoe@AliasDomain.com

  1. You need to change the MX record for this domain to direct the email to Office 365:

AliasDomain-com.mail.protection.outlook.com

After you've done this then all email sent to any of those aliases will be delivered directly to the users in Office 365.

joeqwerty
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Agree with what joeqwerty has suggested. You have to add and verify the domain in Office 365 first. Then update the MX record. Here are steps of adding an alias to a user's Office 365 account.

1.Sign in to the Office 365 portal.

2.In the header, click Admin.

3.Under Exchange Online, click Manage.

4.In the left pane, click Users & Groups, and then click Mailboxes.

5.In the list of users, select the user to whom you want to add an alias email address, and then click Details.

6.Expand E-Mail Options. A list of user's email addresses is displayed.

7.Click Add to add a new alias email address.

For details, you can read this article, How to add an alias to an Office 365 account

Beverly Gao
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