But I still don’t understand really what is happening or how to work around it.
This error <localhost>: Helo command rejected: need fully-qualified hostname; happens sometimes here, but not always. How can this be?
It seems the mail server is sometimes accepting the EHLO from <localhost> and sometimes not? Any idea how this can be? Must be some inconsistency on our mail server? Using postfix here, with same smtp submission settings as shown in this GitHub issue comment here: HELO "localhost" rejected (smtp and submission port) · Issue #1847 · deltachat/deltachat-core-rust · GitHub
Is the proposed solution (sending EHLO [127.0.0.1] instead) already integrated in newer DeltaChat versions, or does it just in the development version? My DeltaChat installs are from December I think.