A customer was using moderated distribution groups and asked if it was possible to disable the expansion of distribution groups in Outlook. As you probably know, after entering a distribution group in the address bar, Outlook will show a “+” sign next to it which will let you replace the distribution group with all its members as recipients. Especially with moderated distribution groups this might be found an issue, because customers might want to hide membership information from senders.
Unfortunately, hiding isn’t a real option because distribution groups are universal distribution groups and its members are readable. Fiddling around with the ACL sounds like a viable option (remove permissions to read members) but sounds like a potential cause for other issues so I wouldn’t recommend that.
What you could do is remove the option to expand the distribution group by setting one of the following a registry keys, depending on your Outlook client version:
Outlook 2013
HKCU\Software\Microsoft\Office\15.0\Outlook\Options\Mail\DisableDLExpansion=1 (REG_DWORD)
Outlook 2010
HKCU\Software\Microsoft\Office\14.0\Outlook\Options\Mail\DisableDLExpansion=1 (REG_DWORD)
Outlook 2007
HKCU\Software\Policies\Microsoft\Office\12.0\Outlook\Options\Mail\DisableDLExpansion=1 (REG_DWORD)
Outlook 2003 or earlier
Outlook 2003 or earlier versions don’t have this option.
After you set this key and restart Outlook, you’ll see the “+” sign is gone:
If you want to roll out this change throughout your organization, use this key in a custom GPO template or utilize the GPO templates that ship with Office (OutlkNN.adm, where NN is determined by the Outlook version), and configure the GPO accordingly.
Be advised that setting this registry key disables expansion for all distribution groups. Specifically disabling it for moderated groups is impossible, since Outlook has – and should have – no knowledge of the type of distribution group.
Note that the distribution group expansion option is something typical of the Windows version of Outlook, since Outlook for Mac nor Outlook WebApp (OWA) have this option.
EighTwOne (821) 
I was not aware of this feature in Outlook. Cool! Another aproach is to use dynamic distribution groups. These cannot be expanded and do not require an all or nothing adjustment to Outlook.
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We have a situation where the + sign allowing expansion is present for some DL’s and not for others. I’d like to expand the ones not showing a +. Any idea how I enable that?
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Could be due to the type of DL (Distribution Group versus Dynamic Distribution Group) or perhaps the DL isn’t contained in the Global Address List (showInAddressBook).
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Thanks for the reply. It is in the GAL. I don’t know how to tell if it’s dynamic. I did wonder if it was security because I can expand a group I’m a member of but not the one I want to of which I’m not a member. As far as I know they are the same type of group.
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dear all
I’m looking for how to disable expand group distribution , a Specific group are there any config ?
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Hi there, I have Outlook 2016 on Win10 with Exchange 2013 CU12 and by default I don’t seem to be able to expand any Distribution lists, i.e. the + sign is not displayed. I would like to make this + sign visible.
I have added the following reg key but no change:
HKCU\Software\Microsoft\Office\16.0\Outlook\Options\Mail\DisableDLExpansion=0
Outlook2016 can see the members of the DL as the little mail tip comes up “This email message will be sent to about ## recipients” and I can also look at the members of the DL via properties in Outlook.
Any assitance would be appreciated.
Cheers
Ben
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The distribution group option is the same for OL2016 – can you verify you are entering a distribution group?
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Hi Michael, apologies for not getting back to you on this one.
Yes I am entering in a DL.
Is there a specific setting to enable DL expansion, as the reg key above doesn’t work for me.
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Okay I worked it out there is another registry key/group policy for Microsoft Office which disables this feature for Outlook 2016:
If disabled or not configured then the option to expand DLs is not displayed:
Group Policy:
User Configuration \Policies\Administrative Templates\Microsoft Office 2016\Manage Restricted Permissions\Always expand groups in office when restricting permissions
Registry:
HKCU\Software\Microsoft\Office\12.0\Common\DRM\AutoExpandDLs REG_DWORD: AutoExpandDLsEnable
(0 = disabled and 1 = enabled)
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Thanks for the heads-up
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