One problem I had with Kmail was that I share an account with my wife, so how could we both see the mail if we logged on as different users and downloaded our mail at different times? Sound confusing? Basically, I needed to set up Kmail where we could both see all the inbound mail and filter it to our own inbox.

Here's how I did it:

(1) Add a folder by selecting Folder >> Create, for each user. I named it after the user(ie.. "From User"), but you can name it whatever name you like.

(2) Name another folder for filtering the mail to the other user (I named it "To User").

(3) Log in as root and set permissions for the "From User" and "To User" for both users. Remember, you will have to set permission to the whole path /home/username/Mail/inbox; that means using chmod rw on each directory.
(In Mandrake 7.1, I right-clicked, selected Properties, and gave read/write access to the group and selected the user as the group)

Sensei's Note:
chmod 664 will do for the example.

It should look something like -rw-rw---- on the each directory/file. I made the mistake of only giving permission only to the inbox, that doesn't help much!

(4) In each user's Kmail, create a local account (I named it after the other User), have it point to the "To User" inbox in the other User's directory. Have the local account store all incoming files in your "From User" inbox.

Now you can see all the mail from the ISP and what the other User has received for you, and you can filter the other user's mail to them. This may seem trivial to some, but I am patting myself on the back.

For those of you that said "What the fsck?" Here's an example:

There's Joe and Blow who have only one email account through their ISP, but they log in at different times and get each other mail. So, Joe Sets up folders "to Blow" and "from Blow" in his Kmail. Blow sets up "to Joe" and "from Blow" in his Kmail.

Once the permissions are setup, Joe creates a local account (in his settings) and points to the folder /home/Blow/Mail/To Joe. Joe then specifies the incoming mail from this folder goes to "From Blow". Got it? Blow will do the same, but with opposite names of course.