When you upgrade to OS X Mavericks, a number of built-in applications will be updated to their latest versions in order to support the various features of the new operating system. One of these is Apple's Mail application, and when upgraded the program will need to update the Mail database in order for its new features to be properly used.
When this happens, you will see a "Welcome to Mail" window appear that instructs you to either continue and download Mail messages, or quit and perform this action later.
This message should only appear the first time you launch Mail, but for some on Mavericks, this message appears every time they open the program.For better or worse, I never saw this message. Perhaps this was because I was working from a sync with my Yahoo! News source. If my Mail database had to be updated, it may well have been happening in the background without my noticing. The only notice I did take occurred as a result of poor responsiveness when I had accidentally deleted a message that had to be recovered from Trash. That message involved a rather long thread and, as I previously reported, managed to take an inordinate amount of time.
Meanwhile, I have yet to see anything about Mail's tendency to crash (or, in the language of Activity Monitor, "not respond") showing no sign of life other than a spinning rainbow. This has happened a couple of times when I happened to have mail with a lot of images that I wanted to print to PDF. The first time I tried using Export to PDF… explicitly, and that was the kiss of death. The second time Mail died just by my requesting Print, without having a chance to say how I was going to use the Print command.
I made the move over to OS X Mail during a period of transition when att.net Mail ("powered by Yahoo!") was in a state of transition that was basically catastrophic for anything trying to get work done. Things are now far more stable on the Yahoo!-powered side, which I can run from an even more stable Firefox. Will someone remind me just what the virtues of Apple software are supposed to be?