Messages not received by member of distribution list

User complains not receiving email sent to DL they are member of. 

Where is DL? On prem exchange. makes note to move all the stupid DLs to 365. User is member of list.

where is user mailbox? somebody elses computer. 

add self to list; emails received. 

check users outlook blocklist. all good.

Explorer tool in o365 is great for quickly confirming delivery. its nice that they changed the default search type from Malware to All Mail, saving a click. Messages sent to DL from outside email address getting all the way through to end users. interestingly, not user. 

Back to on-prem exchange. Lets remove and re-add user to DL. maybe something got borked in AD. 

Delete user from list. good. 

re-add user... where the f did they go? cannot find user in On-prem ECP. no remote mailbox!

Easy to make a remote mailbox with: 
Enable-RemoteMailbox username –RemoteRoutingAddress alias@domain.mail.onmicrosoft.com

ERROR. ExchangeGUID is mandatory. Database is Mandatory. etc. etc.

Lets take a look at the user. Why can i not enable remote mailbox for them? hm, what is this msExchangeHomeServerName attribute? It is referencing a long dead exchange box, and no other active users have the attribute. lets remove it and try enable-remotemailbox again. Success!

now that the remote mailbox is enabled, we need to connect it to the existing o365 mailbox with:

Set-RemoteMailbox username –ExchangeGUID <exchguid>

you can get the o365 GUID with o365 mgmt powershell modules:

Get-Mailbox –Identity emailaddress | fl Identity,ExchangeGUID

Now that the user account and mailbox is repaired, add them to the DL again (on-prem)

Add-DistributionGroupMember -Identity "Staff" -Member "JohnEvans@contoso.com"

My theory is that this occurred because an admin resurrected an old account for this user, who is a long term consultant. They had an on prem mailbox before, but it had been removed. When the user returned, there was no mailbox to migrate so they just made a new o365 mailbox. This mostly worked fine, except that the on-prem DL had the old user (with same name) as a member so it appeared they were added, but of course that identical display name is not the same object as the new o365 user/mailbox.

once the remote-mailbox was enabled for the user, DL emails were received as expected.