Can I use AD synch with Office 365 if I have an on-prem Exchange server?

Hello -

My current environment has a Windows 2008R2 Server running Exchange 2010. Please note that at one point it was incorrectly setup as domain controller in addition to running Exchange.  We want to remove this server.

We also have a separate Windows 2012R2 Domain Controller.  We want to retain this server.

We are also in the process of moving towards hosted Office 365.  We will be doing a cutover rather than migrating.  So we will never be in a hybrid mode.

That said, my question is this.  In the past I have setup new servers with an Office 365 connector (Dir Synch?).  However, I have not done this with a server that has an on-prem Exchange Server.  

The fact that we have an unsupported infrastructure (DC with Exchange) makes me hesitate to introduce something new.  However, I wanted to know if we can use Office 365 Dir Synch when the AD already has an internal Exchange server.

Thanks in advance.

Who is Participating?
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.

Adam BrownSr Solutions ArchitectCommented:
You can connect Azure AD Connect (Which has replaced DirSync) to sync an environment with OnPrem Exchange. Doing so is one of the first steps in the process of building a Hybrid Exchange environment. Having OnPrem Exchange will have no impact on AD Connect syncing data to O365.

You will, however, want to make certain that each user has a valid license attached to their O365 account before you start migrating mail.
realtimerAuthor Commented:
Thank you Adam.

I've already created all my users and groups on O365; and Outlook profiles on my client workstations in preparation.  I have not setup the default domain yet.  
- Would this overwrite everything that I've done?
- Is it an option to setup Azure AD Connect at a later time?
Jose Gabriel Ortega CEE Solution Guide - CEO Faru Bonon ITCommented:
Yeah, you shouldn't have done that in 1st place,
If I were you I would remove everything for o365.
And let the tool do the synchronization for me because it's cleaner, each user have a SID that is unique and it won't match the one in the cloud with the one in the office AD. so it's better to do it good and once.

Basically because if you don't, you should have to do a hard match of SIDs and that's generally more difficult than letting the tool (AD tool) to synch or do the job for you.
Adam BrownSr Solutions ArchitectCommented:
A few years ago, you would have had a *much* more difficult time if you create the users and mailboxes before implementing AD Connect/Dirsync. Now, though, it is less problematic. covers the how and why of this kind of setup fairly well, but the gist of it is that as long as the UPN and primary SMTP addresses of the onprem and O365 accounts match, they should mesh without issues, and the immutableid should also get changed to match the onprem GUIDs of the accounts. If you *do* end up with duplicates, you will need to remove both the duplicate and original and resync, though.

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
realtimerAuthor Commented:
All great feedback.  Thank you so much everyone.  Not sure how I could have given everyone "Best Answer".
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
Microsoft Office

From novice to tech pro — start learning today.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.