While in Calgary for the TNX:TechNet Exchange Tour I was asked a question on Inbox Rules in Exchange Server 2003 and Outlook 2003. The attendee in question wanted to know if there was a maximum limit to the number of Inbox rules you can define in Exchange 2003. I did not think so as I had some 40+ rules defined on my own Inbox and did not have any issues. His Inbox had only 12 rules and he could not create any more. I thought that strange and promised to look into it.
The answer is - there is a limit to the size of all rules in the InBox or any folder in Exchange Server 2003 (as well as previous versions) - 32K. Microsoft KnowledgeBase Article 147298 states:
There is a 32-kilobyte (KB) "packed data" memory limit for the rules on each folder in the Inbox or in a public folder. The reason for the memory limit is that the total sum of the information that is contained in a user's rules must fit in a remote procedure call (RPC).
Sometimes, you may receive the following error message when the number of rules exhausts the 32-KB memory limit:
changes to the rule could not be saved. There is not enough memory or the rules are too complex. Try deleting some rules.
The maximum number of rules depends on the size of the rules that are defined. Additionally, the amount of data that is required for a rule varies depending on the rule. For example, a rule to move messages into a folder for a single recipient takes about 660 bytes. Therefore, there is no defined limit for the maximum number of rules that can be assigned to a folder. Typically, the limit is between 40 and 50 rules per folder.
I was actually hoping for a configurable parameter but the answer is a hard limit. This means that you can have fewer more complex rules per folder or more simple rules, but the size of all rules cannot exceed 32K.