To my suprise, Gnus stopped me from editing and re-sending an email
(I'd forgotten to add an addressee in Cc).
The minibuffer said something like:
rfc2047-encode-region: Invalid data for rfc2047 encoding
In-Reply-To: <
CAOGu_2DQf5dRvsnBQPFiPOxW...@example.net>
("Peter "Lürmer\"'s" message of "Thu, 3 Sep 2020
Once I removed the ü in the above line, the mail was on its way.
Gnus will probably have a knob I can turn, to tell it to allow .. eh,
unicode?