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Changing Email Address Policies in a hybrid configuration

Our default email policy has our domain in all caps (@DOMAIN.COM). When we create a new user and the email gets created in EXO the user shows as mailto:bo@DOMAIN.COM. I know I can go to the email address policy using the on prem exchange server that we are only using for changes (no mailboxes exist on that server). When i click apply is gives me a message about how this will take a long time to apply if you have a lot of users and if you have more than 3000 users to use the EMS (we only have about 500 users).

My concern is, will this start changing things for existing users or removing anything for existing users? For example, will i now see mailto:bob@DOMAIN.COM and mailto:bob@domain.com (as an alias/secondary email)?
ExchangeMicrosoft 365* Exchange Hybrid

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Saif Shaikh

8/22/2022 - Mon
Saif Shaikh

Your email address policy on on-premise has nothing to do with a mailbox creation in EXO. (no mailboxes exist on that server)

Check your default domain in EXO under DNS section. If your default domain is set to bo@DOMAIN.COM then any new user creation will have that email address or will default to bo.

Email address policy will be applied in EXO not from on-premise server.


The domain under DNS in my O365 account is all lowercase. In EXO I don't see an option for email address policies like to do for my on prem server.  My on prem server (which I am only using to create mailboxes and then syncing them to EXO) has the default policy that gets synced when email box are created. That one has the domain in uppercase as well as a few domains we dont use anymore. I was hoping to be able to delete the old domains that we no longer use, convert the one we do use from upper to lowercase and apply the settings. However, I wanted to make sure that wont mess up and existing mailboxes or users.
Saif Shaikh

R u creating a new mailbox directly in O365 portal or do you create it using EAC on the on premise exchange server?

You can use following 2 command using exchange management shell.

Set-RemoteMailbox -Identity [mailbox name] -EmailAddressPolicy $true

Or you can set the proxy addresses after the mailbox creation with

Set-RemoteMailbox -Identity [mailbox name] -EmailAddresses smtp:proxy@emailaddress.com - note the lowercase smtp. If you use UPPERCASE SMTP it will set that as the default email address.

Surely it will impact production since all users mailboxes are tied up to their primary email addresses. Check this article as well which explains to converting-email-addresses-to-lowercase-in-exchange-online-hybrid/  and its impacts.

As per article- if we convert primary email address to lower case meaning make it an alias and use another domain to be the primary email address, then you may have issues with users having to reauthenticate on mobile, or recreate their Outlook profiles. Test these changes first before wider deployment.


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William Peck

Thank you. I have seen the article. We create the users on prem first with a remote mailbox and then when our AD Connect syncs to O365, O365 creates the mailbox.

When I change the email address policy, what happens when i hit apply? Does it impact anything on existing users?
Saif Shaikh

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Saif Shaikh