Migrate and properly store Exchange on-premise inactive mailboxes to Office 365

[Curious_Kevin16]

Hi Guys,

We are planning to move out of Exchange on-premise to O365 (Licenses - O365 E3) and one of the concerns is "Inactive Mailboxes". What is the best way to migrate and properly store these mailboxes in Exchange Online ? 

We thought of migrating the content to Shared mailbox in 365 as they don't require licenses but wondering if this is the best way (we are keen to avoid licenses as much as possible) Also not forgetting the quotas for unlicensed Shared Mailboxes in 365 which is 50GB (at least this is the compliance requirement not a technical cap). 

Appreciate your thoughts ! Thank you heaps !

[#2]

Take a look at the Inactive mailboxes functionality in Exchange Online: https://learn.microsoft.com/en-us/microsoft-365/compliance/inactive-mailboxes-in-office-365?view=o36...

[#3]

Hey Kevin,

What type of mailboxes do you have right know ? And are the mailboxes still online on prem?

Depending on your answer you have multiple options

Mailbox is still „online“ on prem you should change the type to a shared mailbox and to a remote migration to exchange online

If you mailboxes are offline eg exported to pst and located on your on prem file server you can upload those pst files to your tenant and import it to shared mailboxes which lives in exchange online

After that you can decide if you want to stick with shared mailboxes or go the way and use inactive mailboxes wich is a new type in exo. As already mentioned.

Hope this helps

Chris

[#4]

@Curious_Kevin16 

Did your environment in Hybrid mode?

You can consider the below method to move your mailbox to Exchange Online and convert it to shared mailbox 

Move mailboxes between on-premises and Exchange Online organizations in hybrid deployments | Microso...

And yes shared mailbox does not require a license but some limitation:

About shared mailboxes - Microsoft 365 admin | Microsoft Learn

[#5]

Thanks @Kidd_Ip and @Christoph Weste for your response !

We are currently on Exchange On Premise and will fully migrate to Exchange Online soon (cut-over) without Hybrid.

My main concerns are:

1. Mailboxes under 50GB should be fine to convert to a Shared mailbox but how about those that are over 50GB ? (some are even over 100GB as on-premise mailboxes currently got no archive enabled).
2. If we export the mailboxes in to PST files and migrate using a tool, I believe we should have the destination accounts ready with license and a empty mailbox provisioned beforehand. Does this sound about right ?

Again, appreciate your responses !!

Cheers

[#6 · Accepted solution]

@Curious_Kevin16  

Hey,
Hmm good question.
I’m not quite sure but i think you can migrate without an issue even accounts which doesn’t hold a license. If you want/need to use it ( logon to that accounts ) you need a valid license especially if you hit some quota limits.
The question is what do you want to do with that mailboxes. Do you only want them migrated for compliance requests or will they be accessed by real people on regular basis ?

If you need them only for compliance reasons I would change them into inactive mailboxes. For that you need a retention policy which hold your data after deletion. If this is in place you can delete the associated AAD account and the mailbox will become an inactive mailbox which can be „accessed“ via the compliance center.
One thing to consider are auto expanded archives because it is a bit of a „problem“

https://learn.microsoft.com/en-us/microsoft-365/compliance/inactive-mailboxes-in-office-365?view=o36...

Not sure how many big mailboxes you need to migrate. But the following should work without any problems:

Create hold/retention policy.
Assign a valid license and do the migration.
Delete the migrated AAD account this will release the license and „convert“ the the mailbox to an inactive one.
You could do that step by step deepening how short on licenses you are…
Hope this helps

Cheers

Chris

[#7]

Hey Kevin,

1. Mailboxes under 50GB should be fine to convert to a Shared mailbox but how about those that are over 50GB ? (some are even over 100GB as on-premise mailboxes currently got no archive enabled).

Apparently, you need to apply a license to a Shared Mailbox that exceeds 50GB cap. Archiving can be enabled to avoid this but again, online archive requires Exchange Online Plan 2.

2. If we export the mailboxes in to PST files and migrate using a tool, I believe we should have the destination accounts ready with license and a empty mailbox provisioned beforehand. Does this sound about right ?

That's correct. Whether you use the native export/imports methods or a 3rd party tool to import mailbox data, you need the destination accounts to be licensed and a mailbox created beforehand.

Also agree with Christoph on his latest response on this thread. The steps listed by him would be your best bet, given that you wish to store these mailboxes in 365 properly. Alternatively, you can export them as PSTs and store securely in a Azure Storage or a location file server (Wouldn't recommend this option though, due to security concerns).

[#8]

Thanks everyone ! Appreciate your detailed insights here. I think I know what to do now :)

Best Regards
Kevin