SUMMARY
The last couple of days I noticed that the emails I send weren't delivered. I tested back and forth, and could only point the finger at kmail (or akonadi?), as it works fine with my Nextcloud webmail client.

STEPS TO REPRODUCE
1. Compose a mail
2. Try to send it

OBSERVED RESULT
Nothing. No notifications or error messages.


EXPECTED RESULT
The mail was sent with a confirmation notification.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
Operating System: Kubuntu 19.04
KDE Plasma Version: 5.16.4
KDE Frameworks Version: 5.61.0
Qt Version: 5.12.2
Kernel Version: 5.0.0-23-generic
OS Type: 64-bit
Processors: 4 × AMD Athlon(tm) 5350 APU with Radeon(tm) R3
Memory: 15,6 GiB of RAM

ADDITIONAL INFORMATION

I first tried to open kmail from cli. Nothing when sending. Then I restarted akonadi from cli:

org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7f01e42a1dc0) identified as "MessageListTagCacheMonitor - 140276584709584"
org.kde.pim.akonadiserver.search: Executing search "kmail2-48271-SearchSession"
org.kde.pim.akonadiserver.search: Search  "kmail2-48271-SearchSession" done (without remote search)
org.kde.pim.akonadiserver.search: Executing search "kmail2-48271-SearchSession"
org.kde.pim.akonadiserver.search: Search  "kmail2-48271-SearchSession" done (without remote search)
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7f01e425e5b0) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7f01e425e5b0) identified as "SpecialCollectionsMonitor - 93908266813616"
Qt WebEngine seems to be initialized from a plugin. Please set Qt::AA_ShareOpenGLContexts using QCoreApplication::setAttribute before constructing QGuiApplication.
qt.network.ssl: QSslSocket::startClientEncryption: cannot start handshake on non-plain connection
org.kde.pim.akonadiserver.search: Executing search "searchUpdate-1566288213"
org.kde.pim.akonadiserver.search: Search  "searchUpdate-1566288213" done (without remote search)
org.kde.pim.akonadiserver.search: Search update for collection "DeclinedInvitations" ( 41 ) finished: all results:  0 , removed results: 0
org.kde.pim.akonadiserver.search: Executing search "searchUpdate-1566288213"
org.kde.pim.akonadiserver.search: Search  "searchUpdate-1566288213" done (without remote search)
org.kde.pim.akonadiserver.search: Search update for collection "OpenInvitations" ( 40 ) finished: all results:  0 , removed results: 0


Where the interesting line is:

  qt.network.ssl: QSslSocket::startClientEncryption: cannot start handshake on non-plain connection

The smtp setting is set to use STARTTLS with PLAIN authentication. But maybe that's another kinda 'plain'? ;)

Also:

- I tried restarting 'Mail dispatcher Agent'. 'No items in queue' or something like that.

- The composed email is copied to the 'Sent' folder, but of course there are no delivery headers.
- And it isn't in the outbox (local folder).

So it doesn't look like #192046


Oh btw: There was a large update a couple of days ago, with what looked like KDE Frameworks.

A couple of years ago I had to ditch kdepim for Thunderbird, and now after a reinstall I was so glad to be back, as everything seemed to work ok. 
I had used kmail since it started and loved it very much, but I have to say after a couple of years using Thunderbird I never had reasons to file a bug report. Now after max a couple of weeks kmail/akonadi is basically useless to me. So sad :(