So uh. A website I had to sign up for didn't manage to send me registration emails to my own domain (and therefore Hetzner mail servers). Unbox also didn't work. iCloud did work in the end, after some correspondence with their support.
Their emails didn't show up in the mail log, so they never even touched the server on my side...? Their stuff must be quite broken.
Also, they store PWs in plain text. Grand.