IF YOU READ THIS PAGE

This is probably because of inability to relay mail to us. This relay is protected by milter-callback. This means that we don't accept the mail from the relays that don't accept the mail back, to the envelope-from e-mail address. For example, if your MTA says during SMTP_session

    MAIL FROM: <foo@foo.bar>

then the foo.bar MXes will be checked for the existance of the <foo@foo.bar> e-mail address (at least one MX must accept such mail), and/or (depending of the configuration) the sending relay will be checked if it accepts the mail for <foo@foo.bar>.

IMPORTANT

In default configuration milter-callback doesn't accept the mail from submission-only relays, if the IP of such relay isn't declared to be one of the MXes.