Thunderbird has one feature that I would really like to see in Kmail, as it really saves time organizing and finding emails: 
Instead of deleting or saving mails to disk, Thunderbird offers the possibility to "archive" mails with a simple keystroke. When in an arbitrary folder of a mail account , archiving one or multiple mails will move the mails to a special folder in the same mail account. The name/path of the special folder is computed, in case of Thunderbird it is "Archives/${YEAR}" while ${YEAR} is the year of the arrival/sending date of the mail. If the folder "Archives/${YEAR}" does not exist yet, it is created automatically. Using the archiving functionality on multiple mails should compute the destination folder separately for every mail. When working with multiple mail accounts, the mail must be moved to the archive folder of the mail account, that it is stored in, when invoking the archive function. In Kmail, this function should be reachable by a shortcut or a button in the toolbar. 

The feature like described above would be great. To make it more comfortable, I would suggest to add two options in the mail-account settings dialog:
1. choose the base-folder for archiving
2. choose the algorithm for computing the sub-folder in the base-folder, for example:
- "${BASE-FOLDER}/${YEAR}"
- "${BASE-FOLDER}/${YEAR}/${MONTH}"
- "${BASE-FOLDER}/${SENDER-ADDRESS}"

Optionally: Setting a folder as the archive base-folder causes kmail to change the folder's icon, e.g. the icon used for ZIP-files.

Reproducible: Always