<hr>M$ have advised that the rules services of Outlook 2003 crashes the Information stores service on Exchange 5.5.

They have provided a patch, which we'll test and then deploy. The deployment will require a restart of the Information Store service, hence disruption to email services.

I will make scheduled roll-out of the patch on another email. Until then, please ensure no client pc operates Outlook 2003!<hr>
I received the above from our corporate email admin. I will update with any further info as I receive it. In the meantime, I hope this proves helpful.