You should, at a minimum have your ISP defined in your donotsend list. And if you use more than one, put the other(s) in the donotsend list.
For example, I use affinity.com, and comcast.net.
So, my donotsend list looks like this:
AFFINITY.COM
COMCAST.NET
To add items to your 'do not send' list ....
Click 'Configure'
In the box next to the 'Add' button, type in one of the entries from this list, click 'Add', you should see the entry show up in the list.
Repeat until complete.