Version:           1.5.1 (using KDE 3.1.1)
Installed from:     (testing/unstable)
Compiler:          gcc version 3.2.3 20030309 (Debian prerelease)
OS:          Linux (i686) release 2.4.18-bf2.4

When I check for new mail or when KMail runs its periodic check, and when complicated filters are active (e.g. spamc), the program hangs while any filter action is being executed, which may be quite a long time depending on how much processing the filter-activiated program does. Especially when many mail messages one after the other need to be run through such a filter, the program is essentially not usable until the entire check is done. 

For example, I filter (pipe) messages through spamassassin (actually the faster daemonized version, spamc) with razor to weed out the heaps of spam that would normally clog my inbox--such is a critical task for almost all people using email today. Each call to spamc checks an online database of collected spam messages, which may take some time. So, if, say, 50-100 incoming messages are being checked this way, it may take a very long time for all of them to be compared against the database. Meanwhile, KMail hardly responds at all, running in fits and spurts, completely frozen for a few seconds, then active for a split second, then frozen again and so on. It appears as if the main KMail process is waiting for spamc to finish before continuing to the next message and then waiting again, etc. Shouldn't these filter tasks be run in the background, on a different thread of execution, so that users can still use the program (i.e. read already-received mail, organize folders, etc) while the tasks churn away? Really, KMail as it is now is not usable during such lengthy checks. Thanks ahead of time for considering fixing this. I really appreciate the wonderful work you all have done on KMail and KDE in general; overall KMail is an excellent piece of software and I use it every day.