Recipient address rejected: Access denied (554 5.7.1 Error)

EDIT2:

I saw you sent a mail to admin and I tested it from my address and got the same error so it’s not a sympa problem.

Please share your postfix config:

config show postfix

EDIT3:

Is admin@neth.roe3.org entered in the Addresses? (see docs)

Do you use a smarthost?