Outlook 2007 on Exchange 2000 - repeated failed logins accessing

[TMBarnhart]

We are in the process of migrating our users from Exchange2000 to Exchange2007.

99% of my 5000-users are on Outlook2k or Outlook2k3.

Outlook2k and Outlook2k3 can see both the Exchange2k and Exchange2k7
mailboxes.

My 1% on Outlook2k7 love Exchange2k7, but cannot access the mailboxes still
on Exchange2k.

For example, we get a repeated login request (from the Exchange2k7 server)
when they try to access a user's calendar on the Exchange2k server...

I've found numerous mentions of this on the web & forums that discuss it -
but no solutions.

Does anyone have a solution to this...?

 

[TMBarnhart]

Further info about our configs...

1) All of the Exch2k7 CCR-Mailbox Servers and HUB/CAS servers are in the
same domain and same site as the Exch2k servers.

2) All Exch2k7 are at SP1, and Exch2k are at the post-sp3 rollup.

 

[TMBarnhart]

I created another profile for a test account. The test account mailbox was
on the Exch2k server. The new profile for the test account used the same
Outlook2k7 client and could connect to the server with no issues.

so... It is only when the user with Outlook2k7 on Exch2k7 tried to see a
Exch2k user's mailbox - that the repeated logins fail...

A test user w/ Outlook2k7 with Mailbox on Exch2k works correctly and can
even see the original user's (on Exch2k7) mailbox with no login request.