Hi,
I’ve solved this problem by replacing the Reply-To headers with the mailadress of my Nextcloud instance.
See here for full instructions.
[Update 10/2022]
It turns out there is a much simpler way to solve this.
Sharing → Share by mail
And de-activate the option Reply to initiator
In this way the From mailaddres is not changed by Nextcloud and you don’t need to alter your postfix settings.