My previous experiences with spam filtering with KMail have been bad. Its default configuration for Spamassassin is stupid: it pipes message through spamassassin itself, even though the Spamassassin daemon is running and I know it knows its there because it told me it had found it in the spam filtering wizard. The result is that KMail runs very, very slowly when filtering messages. The other annoying thing it does is play the mail notification sound even when the message is spam.
So I spent a little time working this out more carefully and now have the following filters in ~/.kde/share/config/kmailrc. Before describing them, though, the first important thing to note is that I've switched off the "play a sound" option in the "system notifications" section in the KDE control centre.
[Filter #0]
Applicability=0
AutomaticName=false
ConfigureShortcut=false
ConfigureToolbar=false
Icon=
StopProcessingHere=true
accounts-set=
action-args-0=/usr/share/sounds/KDE_Notify.wav
action-name-0=play sound
actions=1
apply-on=check-mail
contentsA=is in address book
fieldA=From
funcA=is-in-addressbook
name=Avoid Filtering Known Senders
operator=and
rules=1
First, if a mail comes from someone in my address book, then play a sound and stop processing filters. This means that I avoid running spamassassin unnecessarily and get useful audio notification.
[Filter #1]
Applicability=2
AutomaticName=false
ConfigureShortcut=false
ConfigureToolbar=false
Icon=
StopProcessingHere=true
accounts-set=111103578
action-args-0=Spam
action-name-0=transfer
actions=1
apply-on=check-mail,manual-filtering
contentsA=YES
fieldA=X-UEA-Spam-Flag
funcA=contains
name=UEA Spam
operator=and
rules=1
Next, (this one is pretty specific to me) if the UEA mail server has assigned a spam flag to the message, send it straight to the spam folder and don't process any further filters. Again, it avoids passing it through Spamassassin and avoids playing a sound.
[Filter #2]
Applicability=0
AutomaticName=false
ConfigureShortcut=false
ConfigureToolbar=false
Icon=
StopProcessingHere=false
accounts-set=
action-args-0=spamc -L
action-name-0=filter app
actions=1
apply-on=check-mail,manual-filtering
contentsA=256000
fieldA=
funcA=less-or-equal
name=SpamAssassin Check
operator=and
rules=1
If the message gets here then it is sent through Spamassassin. By default, KMail piped it through spamassassin, so it was just a matter of changing the argument to "pipe through" to "spamc -L" to make it run a bit faster.
[Filter #3]
Applicability=0
AutomaticName=false
ConfigureShortcut=false
ConfigureToolbar=false
Icon=
StopProcessingHere=false
accounts-set=
action-args-0=Spam
action-args-1=P
action-args-2=R
action-name-0=transfer
action-name-1=set status
action-name-2=set status
actions=3
apply-on=check-mail,manual-filtering
contentsA=yes
fieldA=X-Spam-Flag
funcA=contains
name=Spam handling
operator=or
rules=1
This one KMail set up automatically. It just moves spam flagged messages to the spam folder.
[Filter #4]
Applicability=0
AutomaticName=false
ConfigureShortcut=false
ConfigureToolbar=false
Icon=
StopProcessingHere=true
accounts-set=
action-args-0=/usr/share/sounds/KDE_Notify.wav
action-name-0=play sound
actions=1
apply-on=check-mail,manual-filtering
contentsA=no
fieldA=X-Spam-Flag
funcA=contains
name=Notify Non-spam
operator=or
rules=1
This one I added. If Spamassassin decides the message isn't spam, then play a sound. Now I only get sound for messages from people in my address book and messages which Spamassassin thinks aren't spam.
There are two further "filters" which are not applied to incoming messages but which appear the context menu to help teach Spamassassin about spam. Again, they are created automatically by KMail.
It seems to be working all right so far 
We had a man from ITCS in the Music department yesterday to talk about buying new kit. This is unremarkable except that he had no qualms about admitting, "I don't know the AHRC is". This is indicative of the state of research IT support at UEA; there simply is none.