The developers of our shop software had broken the contact form a long time ago (At least July 2019).
We did not manage to fix it on our own with suggestions by other people using other mail server software.
It was broken for such a long time that we replaced the „contact“ link in the shop footer with our email address.
We just installed the new, quarterly update this weekend and….. it’s finally repaired.
They may have added a new issue, breaking it if non-ASCII characters are entered and they broke the update but at least it works at all.
We changed the footer for „Contact“ to point to the contact form again and added the email link as „Email“, so you can choose both.