Office 365 - Azure AD Connect Sync


I have a question regarding setting up Azure AD Connect between On-Prem AD and an already existing Office 365 tenant.

Some time ago, a customer setup their mailboxes within Office 365. As mentioned above, it is completely independent of their On-Prem domain. There is no synchronisation between their On-Prem domain and Office 365 but there is now a need to install Azure AD Connect to start synchronisation. I found this MS article,, which helps but I have a few questions that I am hoping someone can answer for me.

1. If I test by filtering on a OU with a single account to ensure it works, will all other accounts in O365 get removed or will they be unaffected?  I do not want to test if it going to cause greater problems.

2. I found that the On-Prem accounts still have a value for msExchMailboxGuid. I understand that under initial syncing, mailboxes may not be created. As the mailboxes already exist, would this value prevent O365 from mapping the O365 account to the correct On-Prem account? If it does, would clearing the value help?

Can anyone advise please?

Many Thanks
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

IvanSystem EngineerCommented:

when you sync On-Prem with already created tenant on O365, nothing will happened with O365 users, if they are different.
New user synced from On-Premm will simple be created in cloud, so you can try it with no problem.
if they are the same, then either duplicate will be created or cloud users will be merged and will show as synced, and not created in cloud.

For a test, simple create some new user on-prem and sync it, and it will be synced to cloud. Nothing will happened to cloud users.

In order for you to map On-Prem users with cloud, you need to use SMTP matching. How to do it is written in following post

As you have suggested, first create test account just to test sync for 1 On-Prem user, then test SMTP matching by creating user on cloud and then create same user On-Prem and try to sync them.

Important: If SMTP match fails for a user, it will not pass when you run it again. I did not have this problem, but maybe I was lucky when I did it.


Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
PberSolutions ArchitectCommented:
No comment has been added to this question in more than 21 days, so it is now classified as abandoned.

I have recommended this question be closed as follows:

Accept: Ivan (https:#a42388321)

If you feel this question should be closed differently, post an objection and the moderators will review all objections and close it as they feel fit. If no one objects, this question will be closed automatically the way described above.

Experts-Exchange Cleanup Volunteer
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Office 365

From novice to tech pro — start learning today.