Ink: A Responsive Email Framework from ZURB
Quickly create responsive HTML emails that work on any device & client. Even Outlook.
#newsletter
HTML Email Design Guidelines
From Campaign Monitor. HTML Emails are horrible, as evidenced by all the horrible things going on in this excellent guide.
MailPress
Powerful WP plugin for sending HTML and plain text emails via WP.
HTML Email Guide
A decent guide on things to consider/do when creating HTML email to avoid problems with all the different clients out there.