Exchange 2013 migration to O365, mailNickName attribute duplicates.

Hello Experts. I have exchange 2013 enterprise, with AD 2012 R2. We are planning on migrating to O365. I ran IDfix tool against our on-prem AD for preperation. It found duplicates for attribute: mailNickName. We have muiltiple domains (companies) under us, all in one AD domain, forest. Our policy for mailnickname attribute is, anyting before the '@' in smtp email.

For example for distribution groups: customerservice@toyota.com, and customerservice@honda.com. The mailnickname for both would be 'customerservice'. Which means, there is duplicate. My questions are:

1. What specifically is the mailNickname attribute used for?
2. Will this be an issue with migration to O365, with syncronizing directories?
3. Can we make the mailNickName attribute be the entire smtp email address? This will solve the duplicate issue.
4. Can we make it so that the mailNickName attribute is not synced to the cloud? What effects will this have?

This is seeming to be a big issue for us, due to the number of companies we have under us. Any advice will be really helpful. Thanks in advance.
Newguy 123Asked:
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timgreen7077Exchange EngineerCommented:
1. What specifically is the mailNickname attribute used for?
It's basically the user alias such as the UsersamAccount


2. Will this be an issue with migration to O365, with synchronizing directories?
No this will not interfere with syncing directories.


3. Can we make the mailNickName attribute be the entire smtp email address? This will solve the duplicate issue?
No it's cant be an email address. If these similarities are each in different domains in your forest you should be fine because even though there is that similarity the user SID will still be a distinguisher. These need to be different boundary domains and not just different email address domains.


4. Can we make it so that the mailNickName attribute is not synced to the cloud? What effects will this have?
I haven't done this myself but you can change what attributes are synced using the AD Connect Synchronization Service Manager, but according to below link its really not recommended. review it and see what you think.

https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnectsync-attributes-synchronized
Newguy 123Author Commented:
Thanks for the feedback Timgreen7077. We have a 1 domain on-prem AD. In your answer to question 3 'These need to be different boundary domains and not just different email address domains.', will thid answer still apply? I guess not?
timgreen7077Exchange EngineerCommented:
Since they are not different boundary domains then you will have the duplicates but it will not stop you from syncing the directories. They are basically aliases so you may get an error with the actual sync when synced to O365. I have always corrected these duplicate issues so I'm not 100% how O365 will handle them, but it will not interfere with directory syncs. It will definitely be exclusive to that object. You can give it a different alias but keep the email address if you choose as a proxy address, so just add something different to alias and keep the email address.
I think O365 will still sync the objects but this error is just to make sure that the sync goes as smooth as possible but i believe it will still sync because their are other unique identifiers on objects other than the alias.

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MaheshArchitectCommented:
1 and 4) mailnickname is special exchange attribute and must be synchronized to Azure AD to populate synced objects in Exchange Online GAL as mail enabled contacts (users), without this attribute synced mail enabled users / mailboxes cannot be populated in O365 GAL
Normally in single exchange org, exchange cmdlets hesitate to allow duplication of mailnickname to maintain uniqueness and duplication may happens if attributes are modified with tools other than Exchange shell or console, that is why Microsoft recommends to have exchange server for user object exchange specific attribute management to avoid errors though you don't have exchange as messaging platform onpremise.

2) This should not be an issue as long as mail attribute (Proxyaddreses) is unique

3) That would be an invalid format

More Reading:
https://support.microsoft.com/en-us/help/3190357/how-the-proxyaddresses-attribute-is-populated-in-azure-ad
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