One of the things I’ve found out while migrating to Exchange Online was around access to shared mailboxes, and having to alter my methods slightly.

For Exchange On-Prem (Exchange 2010), whenever I created a shared mailbox I would control access with a security group. If the mailbox was called “Finance”, then I’d have an Active Directory Security Group called “Finance Mailbox Full Access” and give that group full access to the mailbox. Then, I’d add the staff that needed access to that group – meaning it was easier to track and manage who had access to what, particularly with nested group support so I could add a whole department in.

The extra win on this approach was around applying logic to the groups. When someone joined or left Finance, the user management process would ensure the user was added to a group for Finance. That would then feed into all the access that Finance were granted through the groups that single group was inside – and it works great. 

The good news is that this is still possible going to Exchange Online, my nested and automated systems can continue to work. However, some changes were required to make this work.

Firstly, any security group that is going to be added to an Exchange Online mailbox for access must be mail enabled. This goes against my personal best practice as I liked complete seperation of security groups and email groups, because I didn’t want to deal with scenarios where ‘All Finance need this resource, but I don’t want them to get the emails about the same resource’. Still, having a descriptive security group name should avoid that happening. 

This leads to another catch – to mail enable a security group, it has to be ‘universal’. In turn, that means every other group that those groups are inside also need to be universal.

For me this was easy, since all my security groups were cleanly in a single OU, and there was no impact to me on changing them to universal (check this for yourself though!)

Get-ADGroup -SearchBase 
“ou=Security Groups,ou=ABC,dc=com” | Set-ADGroup -GroupScope DomainLocal

After that, ,ail enabling an existing security group is easy with the PowerShell command:

Enable-DistributionGroup

I was actually able to mail enable all my Security Groups that granted mailbox access at once, by getting all the groups and filtering them down to only ones that contained the word ‘Mailbox’

Get-Group -resultsize unlimited | where {$_.name -like “mailbox“} | Enable-DistributionGroup

You’ll also need to mail enable any groups inside the groups, and you should be able to work this out based on the above commands.

Keep in mind you’ll probably want to hide all these groups from the address book so users can’t see them – in Exchange 2010 the GUI lets you mass select and change this option, but it’s also easy to do from PowerShell (but you’d need to 

Get-Group -resultsize unlimited | where {$_.name -like “mailbox“}| Set-DistributionGroup –HiddenFromAddressListsEnabled:$true

Note that if you use PowerShell to give a non mail-enabled Security Group access to a mailbox, it will appear in the Exchange admin center, but it won’t work. As far as EAC knows, it has no members because it’s not mail enabled. I found this out the hard way!