Are you authorized to use securenet for SMTP? I'd check their FAQ for what they mean by the error.
Is "188.8.131.52/24" representing your internal net and *not* your public IP?
.You could be over quota for outbound SMTP at securenet.
. If you are doing SASL/TLS to the outbound you might have problems with the postfix setup.
. Can you send via another outbound server or directly?