If you use Exchange Online in Office 365 together with Outlook you are limited to 64 KB worth of inbox rules (or appx 45 rules).
When you reach the 64 KB limit, you'll be warned that you can't create any more rules. If that happens, you'll have to delete or simplify some of your existing rules before you can create more. Some ways you can reduce the space used by rules are:
A less known tip to obtain more rules than 45 from the 64 KB is to type an alias manually instead of selecting it, when prompted to browse to the desired user alias/DL in the rules wizard.
1. Open the Rules Wizard, and click the "people or public group" link
2. Instead of browsing to a person or group, simply type the alias in the "From ->" field
3. The alias is known to Exchange
If you browse to the alias (instead of typing it manually) Exchange will use the Distinguished Name (DN) which takes up significantly more space, thus limiting the amount of inbox rules you can fit into the 64 KB.
UPDATE september 2012: A recent update made in Office 365 Exchange Online allows Tenant Admins to increase the Inbox Rules Quota limit for their users. The update has been deployed to the Office 365 Service and you can benefit from it today! See this article for more: Tenant Admins can now increase Inbox ‘Rules Quota’ limit for their users!