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.
However, once the new version is installed one should not switch back to
the old version because of changes in the index files and configuration
file. Just to be sure, we recommend that users make a backup of their mail
folder (~/Mail) and compress all folders before switching to the new