Since tha KDE Applications 17.12 update on Archlinux, I cannot sent emails anymore with Kmail, it says "Connection to the serveur interrupted". I did not change anything to my previously working configuration.
On a similar note, using Fedora 27 + 17.12 kmail, I get the following error with smtp.gmail.com: "Failed to transport message. Server error" I can't find any way to display what the error actually is - but this was working prior to the 17.12 update.
*** This bug has been marked as a duplicate of bug 387926 ***