We recently have added a restriction to smtp.iinet.com that the sending email address must be a valid email address on our servers.  Meaning that if you mail software is setup incorrectly and trying to send out using an email address from another mail provider such as gmail.com, icloud.com, outlook.com and many others our server will not allow the message to be send and give an error message.  Please check with the other provider you are trying to send email out using that email address as it is a software configuration problem with your setup with the other email provider.