If KMail does not send mail without saying anything, the “agent” responsible for dispatching messages may be stuck. Of course, you should first verify you have proper network connectivity so mail can be sent!
To remedy this, it might help to abort the current action and restart it. First, quit KMail by using KRunner
(Alt+F2) or Konsole and typing:
kquitapp kmail .
Note that a normal Alt+F4 or → does not do the trick! Wait a minute, then start KMail again. Now start
akonadiconsole using KRunner (Alt+F2) or Konsole. Go to the
Mail Dispatcher Agent (under the Agents tab), do a right-click, and abort the current action.
You will most likely get some error messages popping up. Now go back to KMail and choose → . Now it might work. If not, instead of aborting the current action,
try toggling the offline/online status of the Mail Dispatcher Agent (same context menu) or restarting things
as mentioned in workaround 2 of the previous topic.
akonadiconsole can be quite helpful for a number of situations because it shows all the “agents”, the separate components of the KMail backend. You can stop and start them, put them in offline mode, abort ongoing actions etc.. It can be very helpful when KMail is acting cranky.
Sometimes the Mail Dispatcher Agent fails to function because the dbus daemon (a system-level facility for inter-process communications) is not functioning correctly. Your best bet in this circumstance is simply to reboot the system. The dbus daemon is one of the first processes started when you log in to the KDE desktop, so it cannot be easily stopped and restarted. The whole Plasma™ environment depends on it.