I'm having same problems with my AD infrastructure. I'm using Exchange as email server and when I apply send as to same user by AD , for same user works fine for other not.
First I was thinking that was a problem of exchange , but for the moment i don't think so , I have see same issues in AD , account Unknown in all most all users in the security tab .
I'm a little bit lost, I was waiting same feed back from exchange support but nothing yet. SO I was wondering if you can provide me with same docs in order to run some testing and maintenance to check the consistency of AD to be sure that all its ok there.
Any of your suggestions are really welcome.
Here a detail about the infrastructure:
AD Catalog server 1 : windows 2003 Sp2
AD Catalog server 2 : windows 2003 SP2
Exchange 2007 running CAS + MBX + HUB windows 2003 64Bits SP2
Exchange 2007 running edge + Forefront
Thanks in advance