Misspelled words in KMail 5.4.0 are not getting underlined. Spellcheck works, while manually running spellcheck misspelled words are correctly recognized.
I have the same problem on my system. Automatic spell-checking initially works, after reboot, but then stops working after some time (I do not know what triggers it). Restarting kmail does NOT fix it. OpenSuse Leap 42.2 Kmail 5.3, KDE Frameworks 5.26.0, Qt 5.6.1 I have multiple dictionaries installed.
i see it working as expected in kmail v 5.7 on openSUSE tumbleweed.
Thanks for the update; changing status.