For a POP3 account I set “days to leave messages on the server” to 8 days, but KMail does not delete any e-mails on the server. This happens for example on a Yahoo and GMX server.
Can confirm. Started happening after the update from kmail in KDE Applications 4.14.11 to 17.04.3.
Same problem here with a delay of 30days (POP account) Debian 9 (stretch) : kmail 5.2.3 & akonadi 5.2.2 Maybe this is a duplicate of #375687
*** This bug has been marked as a duplicate of bug 375687 ***