Hi,
I'd like to add something to excellent Joe's reply as I have already had this problem in the field. If your machine don'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 wich port and encryption if needed. Be sure that the 'Device E-mail address' and the 'From' address (they should be the same) is also a valid address 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' in 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 need.
Hope it's help.
Hope you success!
K.