As Fabian Vogt pointed out elsewhere (https://bugs.kde.org/show_bug.cgi?id=387926#c30), ksmtp sends the server hostname as the EHLO domain parameter (confirmed via telnet and wireshark): EHLO mail.snafu.de 550 EHLO/HELO not allowed by local policy. In this case my Provider rejects the EHLO, which results in failure to transport the e-mail. Any other domain will be accepted: EHLO foo.bar 250-sour.ops.eusc.inter.net Hello ... Consequently, this can be worked around by specifying an explicit custom hostname in the SMTP settings e.g. in kmail2. It has been recommended somewhere that in case the client has no (meaningful) domain, the bracketed IP address should be sent, such as: EHLO [nnn.nnn.nnn.nnn] In my case, this is accepted, too. I'm not 100% sure this is a bug, but it seems sane client behavior not to tell the server it ought to talk to itself. It's understandable that the server would test for this domain and none else. This issue didn't arise until a couple of days ago. I do not know what the default EHLO parameter was before that change.
My system is openSUSE Tumbleweed. kmailtransport 17.12.0-1.1-x86_64 ksmtp 17.12.0-3.1-x86_64 (How does that relate to the 'Version' dropdown above?)
Fixed by https://phabricator.kde.org/D9485. It simply does the exact same in ksmtp now as kmailtransport's smtp kio slave used to (it worked, so why change). The version in bugzilla does not match ksmtp as there is no ksmtp component in bugzilla (yet). FYI: Patch got added to KDE:Applications for openSUSE - I assume other distros will pick it soon as well.
Thank you, Fabian! localhost.invalid and foo.localnet make up nice domain names, too. :)