msExchHideFromAddressLists isn't in AD and I can't add it but need to remove people from GAL

Hello @Nick

If these users are synchronized from on-premises Active Directory, that likely explains why EAC returns “Couldn't update mailbox global address list info.” For directory-synchronized recipients, the authoritative Exchange attributes may still be on-premises rather than directly editable in Exchange Online.

Normally, hiding a synchronized recipient is done by setting:

msExchHideFromAddressLists = TRUE

Microsoft documents this attribute as corresponding to the Exchange HiddenFromAddressListsEnabled property.

If msExchHideFromAddressLists genuinely doesn't exist in ADUC Attribute Editor or ADSI Edit, rather than simply being hidden because it has no value, your AD schema may never have been extended with the Exchange schema. I wouldn't recommend manually creating this attribute in ADSI Edit. Schema changes should be handled using Microsoft's supported Exchange schema preparation process after confirming your environment and synchronization configuration.

First, though, check ADUC with View > Advanced Features, open the user's Attribute Editor, select Filter, and make sure Show only attributes that have values isn't enabled. An unset attribute can otherwise appear to be missing.

Also check mailNickname. In synchronized environments, an unset mailNickname can prevent the hide-from-address-list value from having the expected effect after synchronization.

Once the Exchange attribute is available, set it to TRUE, synchronize the directory, and verify in Exchange Online:

Get-Recipient ******@contoso.com |
    Format-List Name,HiddenFromAddressListsEnabled

Microsoft confirms that a recipient with HiddenFromAddressListsEnabled set to True is excluded from the GAL and other address lists.

One additional point: if these employees have actually left the organization and their accounts/mailboxes are no longer required, I'd also review the organization's offboarding process rather than using GAL hiding as the only action.

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