[...] Eventually we found the time to figure out the problem. After some local testing it was clear, that kmail was doing well, and everything else on that system did also (hey it's woody :-) ).
So the only option was the ISP's SMTP, and that was the error, we don't know what went wrong on his side, so we switched over to another SMTP-Relay. Thank you both for your answers, now we have one more happy woody on this planet ;-) Greetings, Marc
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