The issue has cropped up with the Categorizer Queue Length intermittently spiking. First off what is the Categorizer Queue Length?
This counter basically determines the items that reside in the categorizer queue. The categorizer does a few things. It resolves/validates recipients, determines whether the message should be queued for local or remote delivery, expands Distribution Lists (DLs), and detects limits and restrictions.
The first thing to check in order to resolve the spikes, would be that there are ample Global Catalog servers (GCs) and Domain Controllers (DCs) in the environment to perform the look-ups. The best way to check this is on the DSAccess tab in ESM (2k3), or the System Settings tab under the properties of the Exchange server (2k7).
It would also be prudent to run a network diagnostic tool in search of a bottleneck.
Microsoft also recommends monitoring the processor utilization of Inetinfo.exe (categorizer component) and using the e2kdsinteg config object from the ConfigDSInteg tool in order to check for malformed objects that could be slowing the process of directory look-ups. You can get the tool here.
Knowing that the environment probably wasn't the shining star on the top of the Active Directory hill, this was the first thing to check.
The e2kdsinteg log came back with numerous entries for old mail servers, and objects not seen by human eyes in many moons. My advice was to continue to run diagnostics for bottlenecks, and to investigate the purging of these rogue/malformed objects.