Preparing your Exchange migration Part 2 – Converting Shared Mailboxes

My next recommendation prior to migrating to Office 365 is to perform a review of your mailbox types. This is especially important with Exchange 2010 because PowerShell was relatively new back then and there wasn’t a GUI way to create shared mailboxes. I’ve learned over time that many people treat user mailboxes and shared mailboxes as the same, but in an Office 365 world from a licensing perspective they are treated very differently and the price makes it a worthwhile endeavor to do this before the migration.

Office 365 Licensing briefing

I don’t want to spend too much time on this but it’s important to understand  that all users in Office 365 are licensed if they are to have a mailbox and to license a user means you are going to be paying a monthly cost. Shared mailboxes are free in Office 365. They are free in Exchange 2010/2013/2016 as well, but if you mistakenly created a user mailbox and used it as a shared mailbox there wouldn’t necessarily be an up front cost (there should be, but that’s another story that delves into how Microsoft licenses on premise software). Since shared mailboxes don’t consume a license it’s worth it to make the changes before the migration, before you get hit with costs that you don’t need to accrue.

What’s a Shared Mailbox?

A shared mailbox is a mailbox that’s attached to a disabled Active Directory user. That is the big difference, it’s managed by giving other users permissions to it and it’s useful as a replacement for public folders or more generic mailboxes that a whole group of people might manage. Again It does not consume a license.

Reviewing your mailboxes

PowerShell is going to be your friend again here. You are going to want to get all of your mailboxes and recipient type details so you can go to the business and find out if any of user mailboxes can be converted to shared mailboxes. The PowerShell command will look something like

Get-Mailbox -ResultSize Unlimited | Select Displayname,RecipientTypeDetails | export-csv c:\test.csv

I would then take the resulting data and convert it into a table, then sort by RecipientTypeDetails, you will get something like this (this is just a sample, yours might be much larger)

Shared folders blog excel

In the example above, I can identify a few mailboxes that I would consider converting to shared (Sales, Access, Accounts Payable, insurance and licensing). I would then check with the business to see if anyone is using the associated Active Directory accounts for these mailboxes, once you convert them the corresponding AD account will be disabled and not useable.

Converting the User Mailboxes to Shared Mailboxes

Once your research is complete, I would gather all of the mailboxes to convert in it’s own spreadsheet, save it to CSV format and you can use the following command or something similar to convert the mailboxes to Shared Mailboxes.

Import-CSV “C:\alias.csv” | Set-Mailbox -type Shared

What if a mistake is made?

I’ve seen this a few times where you convert and something stops working. If you need to convert a mailbox back you can use the following

Set-mailbox mailboxname –Type regular

This will then re-enable the Active Directory account and set the RecipientDetailsType back to UserMailbox.

Advantages to cleaning up before Migration

  • Doing the leg work upfront helps you to know your system well and takes out the guess work later on.
  • Save on up front licensing costs
  • Might lead you to discovering you no longer need some mailboxes, shrinking your migration will speed it up.
  • Changing the mailbox type after the migration makes the Office 365 conversion a two step process instead of 1. You have to convert the mailbox and then unassign the license.


I am a big believer in doing the leg work up front and reaping the benefits later on. This leg work could save your company significant money in the long run. Say you’ve converted 50 mailboxes to shared. Let’s do the math. $25.30 x 50 user mailboxes x 12 months = $15 180/year. That’s not a small amount.

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