Email Deliverability Brouhaha

Don’t overdo it with HTML. Keep it nice but with content.
Some ISPs block emails based on how many they receive from an address in a period of time, but that’s just part of the game.
http://www.returnpath.com/ is pretty good.
Send the e-mails from different addresses to different batches of your users.
LinkedIn emails get in because they have properly configured e-mail via SPF and DKIM, they are double opt-in, and they somehow work with google and microsoft. (Without double opt-in you get marked as spam more often. – I have no idea whether you already do this.)
Having malformed message-Ids where the domain in the message-ID does not match the sending domain.
Use this: http://designinstruct.com/tool/responsive-html-email–framework-zurb-ink/
Have an easy unsubscribe button.
Avoid link shorteners
Use litmus to test against spam filters
Use mail-tester.com
Compress your images. (kraken.io)
Ask a subscriber to add the email-list address to their address book (right on the subscribe form) and briefly explain why.
Enable a double opt-in subscription process, and send a plain-text confirmation which includes a request to add the email-list address to a recipient’s address book. And, again, briefly explain why.
Useful tool: http://adaptive-images.com/
Use this http://spamcheck.postmarkapp.com/ aggressively.