So we start migration of some 2003 mailboxes to 2010. Keep in mind 2010 SP1 has been configured for weeks with no 2003 mailboxes showing signs of this issue...
So I go to test this in OWA with a test 2010 mailbox. Well I don't see anything wrong every user that should be there according to get-distributiongroupmember -identity "DL-AllHandsNYC" is there? What are the users smoking? OK I pull up Outlook 2010 and yep they are right missing entries. Now normally I would send a test email and see if the missing members get the mail and stop there, but I must make them viewable.
So maybe an issue with 2010 sp1 OAB distribution? Nope!
There are massive numbers of objects with the "hiddenfromaddresslistsenabled" attribute set to $true!!! How has this not been an issue until now?
The behavior was only noticed when our 2010 mailboxes started beta testing the system.
Long story short the fix is easy:
The solution: set "hiddenfromaddresslistenabled" to $false