On starting up Kmail (either cold, from login) or on relaunch in the same session, the Kmail toolbar doesn't appear. I can toggle it on by going to Settings > Show Toolbar (which appears as unchecked). Obviously not critical since things seem to work normally otherwise, but it's slightly annoying. Let me know if I can provide any further information.
Qt version?
(In reply to Antonio Rojas from comment #1) > Qt version? Qt 5.11.1
*** This bug has been marked as a duplicate of bug 395988 ***