Load testing Citrix Server

Hi
This is pretty odd.

We have outlook 2007 on one set of servers (new) and outlook 2003 on the the others (old).

new- 50 instances no problem
old- get around 11-12 and I start getting this:
Unable to open your default e-mail folders. The Microsoft Exchange Server computer is not available. Either there are network problems or the Microsoft Exchange server computer is down for maintanence.

I am using the same id for all tests (multiple instances).

I found one thing that said increase the kerberos token size tried that did not seem to help.

Anyone else ran into this?

Thanks again