Somebody just asked me what I mean when I say "sender." I often refer to senders in contexts like this: A good sender should always do X. Somebody who does Y will always be labeled as a bad sender.
In my mind, a sender is simply somebody who sends email. Someone who causes email to be sent. If you are a list manager, a marketing manager, a list owner, or anything along those lines, you're a sender. If you have a list, people sign up for that list, and you send to that list, you are a sender. I'm a sender, through the list I set up and manage for my friend's jazz club.
On the other side of things, you have "receivers." A receiver is somebody who receives email. AOL and Hotmail and Yahoo are receivers. I tend to use the term "receiver" instead of saying internet service provider, because some sites that handle email only provide email access (webmail, for example), and don't actually provide internet connectivity.
I hope that gives a little more insight as to where I'm coming from when I use terms like sender and receiver.