Mail contact users not appearing in Distribution List - Exchange 2013

We have a Hybrid Exchange environment in-place between Exchange 2013 and Exchange Online.

We have created Mail Contacts on our On-Prem Exchange for users whose mailboxes are in Exchange Online. These contacts have then been added to an On-Prem Distribution List. Whilst they appear in the DL properties, they do not appear when the DL is expanded within Outlook therefore do not receive emails sent to that DL.

I've run:

 [PS] C:\Windows\system32>Get-DistributionGroupMember "DL Name" | where {$_.HiddenFromAddressLis
tsEnabled -eq $true}

To ensure no users are marked as Hidden (various online posts point to this being the issue) and also checked the HiddenFromAddressList properties of the DL in question - this is also set to False as expected.

Running the above command with the $false parameter returns 80 address, expanding the list in Outlook returns 66. Some members are themselves lists, does that account for some discrepancy?

I've checked Azure AD Sync and it's working fine.
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Andy MIT Systems ManagerCommented:
If you look at the list through Outlook Webmail, does it expand correctly in there or does it look the same as when using Outlook itself?
BashContractorAuthor Commented:
@Andy M
It's the same - the contacts are not present when expanded.
Andy MIT Systems ManagerCommented:
Ok, is it the same for a mailbox on the on-prem exchange or does the list show correctly for any mailbox on there?
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BashContractorAuthor Commented:
Okay I have some more info to go on now - the user reporting the issue sent the below NDR received earlier -

 I have also now found that the distribution group exists in both the 'groups' section on both Enterprise and Office 365 ECPs but with different members - the O365 list is the one being used in the Outlook GAL as the people sending to that email both have had their mailboxes migrated to Exchange Online (this list does not have the recipient below listed) - why would that be?

Your message to IMCEAEX-_o=<orgname>_ou=Exchange+20Administrative+20Group+20+28FYDIBOHF23xxxx+29_cn=Recipients_cn=b1f31bcdc9d34150b193fe8db71xxxxx-firstname+20lastname@GBRP123.PROD.OUTLOOK.COM couldn't be delivered.
Your email program is using outdated address information for IMCEAEX-_o=<orgname>_ou=Exchange+20Administrative+20Group+20+28FYDIBOHF23xxxx+29_cn=Recipients_cn=b1f31bcdc9d34150b193fe8db71xxxxx-firstname+20lastname@GBRPxxx.PROD.OUTLOOK.COM
ITservicemanagement       Office 365       IMCEAEX-_o=TDWI_ou=E. . .
Action Required             Recipient

Outdated To address             

Error Details
Reported error:      550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup
DSN generated by:
BashContractorAuthor Commented:
Is the difference because on-prem those 'missing' users who are in the on-prem list are in as mail contacts whereas in the cloud they exist as mailboxes and therefore will not be seen as contacts and need adding in as mailboxes?
BashContractorAuthor Commented:
Just to advise - the business decided they want a new DL creating on O365 with the relevant mailbox users added.  It was the fact that existing O365 mailbox users were created as On-Premises Mail Contacts and the users mailing the list were O365 mailbox users, therefore picking up the reduced count DL that existed in O365 as the On-Prem mail contacts were not in the O365 list.

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BashContractorAuthor Commented:
It achieved the requirement.
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