Murphy's law and it's corollaries dictate that not only will things that can go wrong will go wrong, but they will do so at the least possible convenient time. Thus it was that at the end of April as I was getting ready to transition to a few months of full-time travel Apple's Mail app decided to head south. The main symptom was an incredible use of memory. I could launch Mail and a short time later not only would all 8GB of RAM in my machine be swallowed up but it would be using 20GB more on disk for swap space. All and all a totally unusable situation. Read more about Benefits from the OS X 10.6.4 update