Hi @Benjamin0, thanks for messaging the Community.
It looks like you are currently using a relay address. This happens when you create a new Dropbox account on an iOS device, using the Sign in with Apple button, and then at sign up choosing the option "Hide my Email address".
This is normal behavior when using that service. While Apple will relay any messages sent to the address listed on your Dropbox account back to your real one, the one provided on your Dropbox account may appear unknown or in a strange format.
If you have any further queries, feel free to message back.