The file contains an |exit 100 code

id:content-exit100

You have set up this file to bounce incoming mails. But it is better to reject mails in the first place, before making them bounce back to the sender.

You should check if you could just remove this mailaddress from your configuration so it is properly rejected. If this is not possble (maybe because of a catchall) you should set up a mailbox with a Sieve script to reject mails.

Error

Error: This configuration is probably not working as expected in the standard mail system. This is why you should recreate this mailaddress yourself using the uberspace mail commands.