A0067: There have been instances of network abuse involving mail sent out by web servers. In most cases, unrecognizable commands are sent as part of the SMTP session. A real MTA never sends out such invalid commands. Exim allows a few unrecognized commands in a session to permit humans who are testing to make a few typos (it responds with a 5xx error). However, if Exim receives too many such commands, it assumes that it is dealing with an abuse of some kind, and so it drops the connection.