If you send a lot of email marketing, you should start thinking about authentication.
Authenticating your email is basically like having a license plate on a car. If you do something wrong, or just act suspicious while driving, the police can look up the license plate number, and see if you’re really the owner.
Some ISPs are already checking for these "license plates" on incoming email.
If you do something spammy (like suddenly send waaaaaay too many emails at one time from a fresh new IP address) they’ll check for authentication to see if you’re legit.
There are basically two major types of authentication. Microsoft has their SenderID framework, which is fairly easy to implement. This is very similar to the license plate analogy. Yahoo came up with DKIM authentication, which is kind of like checking a car’s license plate, plus the driver’s fingerprints. It’s more thorough, but also a lot harder to implement for some people.