If you send any bulk email at all – by which we mean mailing list email, or many copies of identical or nearly-identical email sent to many different people – then you really need to be signed up for as many ISP Feedback Loops (“FBLs”) as you can.
Here is a list of available feedback loops as of the time of this writing.
List of Available Feedback Loops from ISPs and Inbox Providers
Ask.com (excite, this is going away)
Yahoo for DKIM signed mail (no longer accepting updates or new
Outblaze (for Confirmed opt-in mail only)