Checking the status of your IP:
These resources allow you to check the status of your IP and if you are on any blacklists.
note: If you find yourself on a blacklist, and they ask you to pay for removal, think twice. In most situations, any blacklist that requires payment for removal carries very little weight in the industry.
Checking the quality of your emails:
These resources allow you to check the "spammyness" of your emails to improve delivery to the inbox.
Articles on best practices:
Articles on email delivery education:
The laws and email delivery:
Getting in touch with email service provider postmaster services:
Understanding Gmail's Tabbed Inbox: