Invalid message-id in mails sent with the internal ISPConfig mail library
Message id from mails from the billing module are faulty (like: Message-ID: <mxwet0.yxj8ml@>). This is not good because spamassassin will flag it with INVALID_MSGID giving it more spam score.
See http://en.wikipedia.org/wiki/Message-ID for how its done.
I have not tested if this message-id thing is deeper down into ISPConfig