I'd like to add something to Joe's excellent reply as I have already had this problem in the field. If your machine doesn't have any authentication and/or encryption in the smtp setting, you could try to put one before going anywhere else.
As Joe said, you will need an authenticated user and password from your domain and you will need to identify which port and encryption if needed. Be sure that the 'Device E-mail address' and the 'From' address (they should be the same) are valid addresses from your domain. Here's some pictures to show you:
You may need to add your domain name before the user: your_domain\valid_user_name
Make sure that it is the same address that you put in 'Device E-mail Address', see the first picture.
Some SMTP Server setup will only permit sending mail without authentication inside the domain. In doubt, telnet your smtp server from a workstation, it will tell you what it needs.
Hope it's helpful.
Hope you success!