Since migrating to Exchange 2010 we have encountered two distinct problems relating to the same issues.
1) Periodically, users are unable to expand group distribution lists and they receive the Outlook error "Cannot perform the requested operation. The command selected is not valid for this recipient. The connection Microsoft Exchange is unavailable. Outlook
must be online or connected to complete this action." If I check the connection status of Outlook, I see that "Directory" is shown as disconnected and will not reconnect.
2) When adding a new account to Outlook during a new user setup on a computer, the new e-mail account wizard is unable to resolve the username stating "Outlook cannot log on. Verify you are connected to the network and are using the proper server and mailbox
name. The connection to Microsoft Exchange is unavailable. Outlook must be online or connected to complete this action."
Both of these issues are linked to the same problem - Outlook being unable to properly establish a connection to the Exchange server. The problem follows the user even to a different computer, but will eventually resolve itself after waiting long enough, usually
about 24 hours. It happens inside the network and doesn't matter if Outlook is trying to connect via RPC or HTTPS. It's intermittent and can't be tied to any particular account.
Our configuration is as follows:
(1) Barracuda Load Balancer 340
(2) CAS/HT servers running Exchange 2010 w/SP1
(2) Mailbox servers running Exchange 2010 w/SP1
Outlook 2007 clients wSP2