If you did not heed our warning from six months ago and are still using KMail < 126.96.36.199 (from KDE 1.x), you are advised to please upgrade to KDE 2.2 (or KDE 2.1.x) immediately. In about a week the bizarre KMail 1,000,000,000th second bug will strike, which can cause corruption of your mail folders. Details about the bug are available here.
The KMail developers recommend that users update to KDE 2.2, which includes a newer KMail with many other bugs fixed and many new features. According to KMail developer Daniel Naber,
The new version of KMail will use old mail files without problems.